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OF-    THE    GEOLOGHOAL 


HISTORY    OF    ORGANISMS 


FOTINDEIJ     BY 


Charles  Elias  Hardy 


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Digitized  by  the  Internet  Archive 

in  2007  with  funding  from 

Microsoft  Corporation 


http://www.archive.org/details/developmentoffroOOmorgrich 


THE  DEVELOPMENT  OF  THE  FROG'S  EGG 


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THE  DEVELOPMENT  OF  THE 
FROG'S   EGG 


AN   INTRODUCTION   TO 

EXPERIMENTAL  EMBRYOLOGY 


BY 

THOMAS    HUNT  (morgan,   Ph.D. 

PROFESSOR  OF  BIOLOGY,   BRYN  MAWR  COLLEGE 


O'^' 


THE    MACMILLAN   COMPANY 

LONDON:  MACMILLAN  &  CO.,  Ltd. 
1897 

All  rights  reserved 


COPVKIGHT,   1897, 

By  the  MACMILLAN  COMPANY. 


Norfajooti  iPress 

J.  S.  Gushing  &  Co.  -  Berwick  &  Smith 
Norwood  Mass.  U.S.A. 


PEEFACE 

The  development  of  the  frog's  egg  was  first  made  known 
through  the  studies  of  Swammerdam,  Spallanzani,  Rusconi, 
and  von  Baer.  Their  work  laid  the  basis  for  all  later  research. 
More  recently  the  experiments  of  Pfiiiger  and  of  Roux  on  this 
egg  have  turned  the  attention  of  embryologists  to  the  study 
of  development  from  an  experimental  standpoint.  Owing  to 
the  ease  with  which  the  frog's  egg  can  be  obtained,  and  its 
tenacity  of  life  in  a  confined  space,  as  well  as  its  suitability  for 
experimental  work,  it  is  an  admirable  subject  with  which  to 
begin  the  study  of  vertebrate  development. 

In  the  following  pages  an  attempt  is  made  to  bring  together 
the  most  important  results  of  studies  of  the  development  of 
the  frog's  egg.  I  have  attempted  to  give  a  continuous  account 
of  the  development,  as  far  as  that  is  possible,  from  the  time 
when  the  egg  is  forming  to  the  moment  when  the  young  tad- 
pole issues  from  the  jelly-membranes.  Especial  weight  has 
been  laid  on  the  results  of  experimental  work,  in  the  belief 
that  the  evidence  from  this  source  is  the  most  instructive  for 
an  interpretation  of  the  development.  The  evidence  from  the 
study  of  the  normal  development  has,  however,  not  been  neg- 
lected, and  wherever  it  has  been  possible  I  have  attempted  to 
combine  the  results  of  experiment  and  of  observation,  with  the 
hope  of  more  fully  elucidating  the  changes  that  take  place. 
Occasionally  departures  have  been  made  from  the  immediate 
subject  in  hand  in  order  to  consider  the  results  of  other  work 
having  a  close  bearing  on  the  problem  under  discussion.  I 
have  done  this  in  the  hope  of  pointing  out  more  definite  con- 
clusions than  could  be  drawn  from  the  evidence  of  the  frog's 
egg  alone. 

In  treating  the  general  problems  of  development,  I  have  tried 
to  keep  as  near  to  the  evidence  as  possible.     I  have  intention- 


81684 


Yi  PREFACE 

ally  avoided  at  times  the  discussion  of  the  more  theoretical 
problems  arising  from  the  experiment,  for  it  seems  to  me  that 
such  discussions  are  out  of  place  in  a  volume  of  this  sort.  Only 
the  early  stages  of  the  development  have  been  considered, 
because  almost  all  of  the  experimental  work  on  the  frog's  egg 
has  been  done  on  the  early  stages,  and  also  because  I  am  more 
fam,iliar  with  the  development  and  with  the  experiments  of  this 
period.  Moreover,  the  later  stages  have  been  recently  most 
admirably  described  by  Marshall  in  his  Vertebrate  Embryology. 

A  few  words  of  personal  explanation  may  be  added.  For 
several  years  I  have  been  collecting  the  material  for  the  present 
volume,  but  as  the  literature  is  so  extensive  and  as  I  have  had 
other  work  to  do  first,  I  made  but  slow  progress.  In  the 
summer  of  1893  I  set  seriously  to  work,  and  owe  much  to  the 
admirable  facilities  offered  by  the  University  of  Berlin.  I  take 
pleasure  in  acknowledging  my  indebtedness  to  Geheimrath 
Professor  Fr.  E.  Schulze  for  many  privileges  and  kindnesses 
extended  to  me  in  Berlin.  The  work  Avas  continued  irregu- 
larly during  the  winter  of  1893-1894  while  enjoying  the  oppor- 
.tunities  of  the  Stazione  Zoologica  in  Naples.  During  the 
winter  of  1894-1895  the  material  was  brought  together  and  in 
the  summer  of  1896  at  Zurich  the  manuscript  was  almost  com- 
pleted. I  gladly  take  this  opportunity  to  thank  Professor 
Arnold  Lang  for  many  courtesies  extended  to  me  during  two 
visits  to  Ziirich.  Dr.  Driesch  has  most  kindly  looked  over 
some  of  the  chapters,  and  has  made  many  valuable  sugges- 
tions. Dr.  H.  H.  Field  has  also  examined  a  part  of  the 
manuscript  and  helped  me  in  several  directions.  To  Professor 
E.  B.  Wilson  I  am  under  heavy  obligations,  and  owe  much 
to  his  valuable  suggestions  and  corrections.  To  Dr.  H. 
Randolph  I  owe  a  debt  of  gratitude  for  kindly  advice  and 
criticism.  I  am  also  greatly  indebted  to  Professor  Joseph  W. 
Warren  and  to  Professor  E.  A.  Andrews  for  advice  in  con- 
nection with  the  revision  of  the  proof. 


CONTENTS 

PAGE 

Introduction xi 

CHAPTER  I 

The  Formation  of  the  Sex-cells 1 

Spermatogenesis. 

"  Direct "  Division  of  the  Germ-cells. 

Oogenesis. 

Comparison  of  Spermatogenesis  with  Oogenesis. 

CHAPTER  n 

Polar  Bodies  and  Fertilization 15 

Extrusion  of  the  First  Polar  Body  and  Egg-laying. 
The  Jelly  of  the  Egg,  and  the  Second  Polar  Body. 
Entrance  of  Spermatozoon  and  Copulation  of  Pronuclei. 

CHAPTER  ni 

Experiments  in  Cross-fertilization 26 

Experiments  of  Pfliiger  and  of  Born  on  Frogs'  Eggs. 
Experiments  on  Other  Forms. 
Experiments  of  Rauber  and  of  Boveri. 

CHAPTER  IV 

Cleavage  of  the  Egg .        .32 

Normal  Cleavage. 

Correspondence  of  the  First  Cleavage-plane  and  the  Median-plane 

of  the  Embryo. 
Roux's  Experiments  with  Oil-drops. 
Historical  Account  of  the  Cleavage  of  the  Frog's  Egg. 

vii 


viii  CONTENTS 

CHAPTER  V 

PAGE 

Early  Development  of  the  Embryo 50 

The  Blastopore. 

External  Changes  after  the  Closure  of  the  Blastopore. 

CHAPTER  VI 

Formation  of  the  Germ-layers 63 

His's  Experiments  with  Elastic  Plates. 
The  Formation  of  the  Embryo  by  Concrescence. 
The  Formation  of  the  Archenteron. 
The  Overgrowth  of  the  Blastoporic  Rim. 
The  Origin  of  the  Mesoderm. 

Different  Accounts  of  the  Origin  of  the  Archenteron  and  Meso- 
derm. 
Later  Development  of  the  Mesoderm  and  Origin  of  the  Notochord. 

CHAPTER  VII 
The  Production  of  Abnormal  Embryos  with  Spina  Bifida     .      75 

CHAPTER  VIII 

Pfluger's  Experiments  on  the  Frog's  Egg 81 

The  Effect  of  Gravity  on  the  Direction  of  the  Cleavage. 

The   Relation   of  the  Planes  of   Cleavage  to  the  Axes  of  the 

Embryo. 
Conclusions  from  the  Experiments. 

CHAPTER  IX 

Experiments  of  Born  and  of  Roux 90 

Changes  that  take  Place  in  the  Interior  of  the  Egg  after  Rotation. 
The  Cleavage  of  the  Egg  in  a  Centrifugal  Machine. 

CHAPTER  X 

Modification  of  Cleavage  by  Compression  of  the  Egg  .        .      95 
Effect  of  Compressing  the  Segmenting  Egg  between  Parallel  Plates. 
Conclusions  from  the  Experiments. 
The  Distribution  of  the  Nuclei  in  the  Compressed  Egg. 


CONTENTS  ix 

CHAPTER  XI 

PAGE 

The  Effect  of  Injuring  One  of  the  First  Two  Blastomeres     106 
Roux's  Experiment  of  "  Killing "  One  of  the  First  Two  Blasto- 
meres. 
Further  Experiments  by  Others  (Hertwig,  Endres  and  "Walter, 
Schultze,  Wetzel,  Morgan). 

CHAPTER  Xn 

Interpretations  of  the  Experiments;  and  Conclusions   .        .     123 
Roux's  Mosaic  Theory  of  Development. 
Theory  of  Driesch  and  of  Hertwig  of  the  Equivalency  of  the  Early 

Blastomeres. 
Roux's  Subsidiary  Hypothesis. 
Experiments  on  Other  Forms. 
General  Conclusions. 

CHAPTER   XIII 

Organs  from  the  Endoderm 137 

The  Closure  of  the  Blastopore,  and  the  Formation  of  the  Neuren- 

teric  Canal. 
The  Digestive  Tract  and  the  Gill-slits. 

CHAPTER    XIV 

Organs  from  the  Mesoderm 146 

The  Mesodermic  Somites. 
The  Heart  and  Blood-vessels. 
The  Pronephros. 

CHAPTER   XV 

Organs  from  the  Ectoderm 159 

The  Central  Nervous  System. 

The  Eyes. 

The  Ears. 

The  Nerves. 

The  Appearance  of  Cilia  on  the  Surface  of  the  Embryo. 


CONTENTS 


CHAPTER   XVI 

PAGE 

Effects  of  Temperature  and  of  Light  on  Development         .    168 


APPENDIX 171 

LITERATURE 173 

INDEX 187 


INTRODUCTION 

The  eggs  of  most  of  our  species  of  frogs  are  laid  in  the 
spring.  In  some  cases  they  are  set  free  almost  immediately  on 
the  emergence  of  the  frogs  from  their  winter  sleep ;  in  other 
cases  the  eggs  are  not  laid  until  some  weeks  or  even  months 
after  the  frogs  have  awakened.  In  almost  every  instance  the 
eggs  are  deposited  in  water  and  usually  in  quiet  pools  or  ponds, 
or  in  protected  bays  along  streams  where  the  water  has  backed 
up  and  has  come  to  rest.  Sometimes  the  bunches  of  eggs  are 
stuck  to  sticks,  grass,  submerged  sedge,  or  even  to  stones ;  in 
other  cases  the  bunches  are  not  fastened. 

The  copulation  precedes  and  lasts  through  the  laying-period  ; 
a  single  male  fertilizing  all  the  eggs  laid  by  one  female.  The 
sperm  pours  out  of  the  cloaca  of  the  male  at  the  moment  when 
the  eggs  pass  out  of  the  female. 

Both  the  male  and  the  female  sexual  products,  the  eggs  and 
spermatozoa,  are  ripened  during  the  summer  and  autumn  of 
the  year  preceding  the  deposition  of  the  eggs,  —  at  least  this 
is  the  more  usual  process.  The  origin  of  these  sexual  cells 
must  first  be  studied  in  order  to  more  fully  understand  their 
relation  to  each  other,  and  the  part  they  play  in  the  subsequent 
development. 


DEVELOPMENT  OF  THE  FEOG'S  EGG 

CHAPTER   I 

THE   FORMATION  OF   THE   SEX-CELLS 

The  development  of  the  sex-cells  is  generally  divided  into 
three  periods  :  1)  a  multiplication-period,  during  which  the 
primitive  germ-cells  pass  through  a  large  number  of  divisions ; 
2)  a  growth-period,  in  which  the  primitive  germ-cells,  that 
have  become  reduced  in  size  through  repeated  division,  grow 
larger ;  3)  a  maturation-period,  when  only  two  divisions  take 
place,  between  which  the  nucleus  does  not  pass  into  a  resting- 
stage.  At  the  end  of  this  last  division  the  male  germ-cells 
undergo  histological  changes  by  which  they  become  trans- 
formed into  spermatozoa.  1 

Spermatogenesis 

The  changes  that  take  place  in  the  testes  of  the  frog  have 
not  been  so  fully  worked  out  as  in  some  other  animals  ;  we 
may  therefore  follow,  first,  the  method  of  development  of  the 

1  This  is  a  modification  of  the  terminology  of  v.  la  Valette  St.  George,  whose 
nomenclature  of  spermatogenesis  is  still  often  used.  La  Valette's  classification 
is  as  follows  :  — 

The  primordial  germ-cells  give  rise  to  spermatogonia,  which  cease  to  divide 
after  a  time  and  increase  in  size.  Each  spermatogonium  is  thus  converted  into 
a  primary  spermatocyte.  Each  primary  spermatocyte  divides  into  two  cells,  the 
spermatocytes  of  the  second  order,  and  each  of  these  divides  once  more,  with- 
out a  resting-period,  to  form  two  spermatids.  In  this  way  four  spermatids  are 
formed  from  each  primary  spermatocyte.  Each  spermatid  is  then  changed 
directly  into  a  spermatozoon. 

B  1 


2         DEVELOPMENT  OF  THE  FROG'S  EGG      [Ch.  I 

spermatozoon  in  two  forms  in  which  the  process  is  better 
known,  and  then  consider  the  special  case  of  the  frog. 

The  development  of  the  spermatozoa  of  Gryllotalpa,  the 
mole-cricket,  has  been  described  by  vom  Rath  ('92,  '95).  As 
the  process  of  spermatogenesis  is  relatively  simple  in  this  form, 
and  as  it  is,  according  to  vom  Rath,  much  like  the  process  that 
takes  place  in  the  frog,  we  may  therefore  first  briefly  consider 
the  changes  in  Gryllotalpa. 

First  Period,  A  cell  in  the  resting-stage  at  this  time  shows 
a  large  nucleus  with  a  distinct  membrane  enclosing  a  network 
of  fine  chromatin  (Fig.  1,  A).  The  beginning  of  the  cleavage 
is  indicated  by  the  withdrawal  of  the  chromatin  from  the 
nuclear  membrane  and  the   thickening   of   the   fibres   of   the 


A  B  C  D 

Fig.  1.  —  Division  of  sperm-mother-cells  in  Gryllotalpa.     (After  vom  Rath.) 

chromatic  network.  The  tangled  mass  of  threads,  or  net- 
work, then  takes  a  somewhat  excentric  position.  This  thread 
seems  to  consist  of  linin,  on  which  chromatin-granules  are 
arranged.  Sometimes  the  thread  can  be  seen  to  be  split  along 
its  length  into  two  parts.  The  halves  of  the  thread  remain, 
however,  closely  sticking  to  each  other.  The  double  thread 
then  breaks  up  by  cross-division  into  twelve  equal  segments, 
or  chromosomes  (Fig.  1,  B).  The  chromosomes  next  become 
shorter,  and  finally  spherical,  and  come  to  lie  in  an  equa- 
torial plate  (Fig.  1,  C).  When  the  chromatin  is  still  in  the 
skein-stage,  two  minute  bodies  are  seen  in  the  protoplasm  just 
outside  of  the  nuclear  membrane  (Fig.  1,  B).  These  are  the 
two  centrosomes,  which  separate  more  and  more  from  each 
other,  and  finally  come  to  lie  on  opposite  sides  of  the  nucleus. 
A  protoplasmic  spindle  develops  between  the  two  centrosomes 
(Fig.  1,  C)  and  the  fibres  of  the  spindle  become  fixed  to  the 


Ch.  1] 


FORMATIOX  OF   THE   SEX-CELLS 


chromatin-granules  of  the  equatorial  plate.  Each  of  the  twelve 
chromatin-granules  divides  into  two  equal  parts  and  the  halves 
migrate  toward  one  or  the  other  of  the  centrosomes  (Fig.  1,  D). 
The  cell-protoplasm  next  divides  into  two  parts,  so  that  two 
new  cells  are  formed.  Each  cell  contains  twelve  chromosomes. 
In  this  way  the  primitive  sperm-cells  continue  to  increase  in 
number  by  a  series  of  cell-divisions,  all  like  that  just  described. 


Fig.  2.— The  formation  of  spermatozoa  in  Gryllotalpa.    The  two  maturation-divi- 
sions.    (After  vom  Rath.) 


Second  Period.  A  period  of  rest  then  follows,  during  which 
the  cells  grow  larger.  During  this  time  the  chromatin  is  again 
arranged  in  a  fine  network. 

Third  Period.  Two  successive  and  most  peculiar  cell- 
divisions  now  take  place.  The  chromatin-network  becomes 
thicker,  and  forms  a  tangled  skein  of  threads  (Fig.  2,  A,  B). 


4         DEVELOPMENT  OF  THE  FROG'S  EGG      [Ch.  I 

Each  thread  is  split  longitudinally  into  two  parts.  Two  cen- 
trosomes  again  appear.  The  chromatin-thread  next  breaks  up 
into  six  bent  rods  or  chromosomes  (Fig.  2,  C).  There  is  some 
doubt  as  to  the  way  in  which  the  next  change  is  brought 
about.  The  account  of  vom  Rath,  which  we  follow  here,  seems 
to  be  in  harmony  with  the  process  that  is  known  to  take  place 
in  some  other  forms  during  this  period  of  development  of  the 
germ-cells.  It  appears  that  the  halves  of  each  of  the  six  bent 
rods  begin  to  separate  from  each  other  except  at  the  ends 
of  the  rods,  where  the  halves  remain  united.  Each  rod  is 
in  this  way  converted  into  a  ring  (Fig.  2,  D).  These  rings 
are  often  so  bent  on  themselves  that  they  form  a  loop.  The 
six  chromatin-rings  lie  close  to  the  periphery  of  the  nucleus. 
The  rings  contract  and  become  smaller  and  thicker  (Fig.  2,  E). 
This  stage  lasts  but  a  short  time  and  is  succeeded  by  a  stage 
shown  in  Fig.  2,  F,  G.  Out  of  each  ring  four  star-like  granules 
are  formed,  the  tetrad  or  "  Vierer-gruppe."  The  four  granules 
of  each  set  are  closely  held  together  by  clear  linin  threads.  If 
each  granule  be  counted  as  a  distinct  chromosome,  then  there 
are  present  at  this  time  six  groups  of  four  chromosomes  each,  or 
twenty-four  chromosomes.  These  twenty-four  chromosomes  be- 
come attached  to  the  fibres  of  the  achromatic  spindle  (Fig.  2,  H) 
and  arrange  themselves  into  an  equatorial  double  plate.  Then 
twelve  of  these  granules  united  in  pairs  wander  toward  one  pole 
of  the  cell  and  twelve  toward  the  other  pole,  and  the  division 
of  the  cell  takes  place  (Fig.  2,  I).  This  process  is  spoken  of  as 
the  first  maturation-division.  Without  passing  into  a  resting- 
stage^  a  second  division  of  each  cell  follows  (Fig.  2,  J).  A  new 
karyokinetic  spindle  is  formed  and  the  twelve  chromosomes 
are  separated  into  two  plates  of  six  chromosomes  each,  which 
go  to  their  respective  poles.  Each  of  the  two  new  cells  con- 
tains therefore  only  six  chromosomes  (Fig.  2,  K).  The  number 
of  chromosomes  is  now  reduced  to  half  the  normal  munher 
present  in  the  other  cells  of  the  body  of  the  animal.  Each  of 
the  four  cells  formed  by  these  two  consecutive  divisions  of  the 
sperm-mother-cell  then  differentiates  into  a  spermatozoon  (Fig. 
2,  L).  Each  spermatozoon  consists  of  three  parts,  —  a  head,  a 
middle  piece,  and  a  tail.  The  head  is  formed  almost  entirely 
out  of  the  nucleus  of  the  parent-cell  of  the  spermatozoon,  as 


Ch.  1]  FORMATION   OF   THE   SEX-CELLS  5 

seen  in  Fig.  7,  A,  B,  C.  It  is  probable  that  a  very  thin  layer 
of  cytoplasm  covers  the  outer  surface  of  the  head.  The  chro- 
matin is  densely  packed  into  the  head-piece,  and  cannot  be 
resolved  into  its  component  chromosomes.  The  middle  piece 
lies  just  back  of  the  head.^  In  some  animals  this  middle  piece 
is  known  to  enter  the  egg  with  the  spermatozoon  and  a  part  of 
it  becomes  the  centrosome,  which  then  divides  into  two  centro- 
somes  and  around  these  arise  the  achromatic  rays  of  the  dividing 
egg.  The  tail  of  the  spermatozoon  is  generally  described  as 
coming  from  the  C3'toplasm  of  the  cell. 

The  development  of  the  spermatozoon  in  the  salamander 
has  been  carefully  studied  by  Flemming  ('87),  vom  Rath 
('93),  Meves  (96),  and  others.  There  are  certain  remark- 
able processes  that  take  place  in  the  spermatogenesis  of 
these  Amphibia  that  seem  to  occur  also  in  the  frog,  but  as 
they  have  not  been  as  carefully  worked  out  in  the  latter  form 
we  may  examine  first  the  changes  that  take  place  in  the  sala- 
mander. Each  year  after  the  male  has  lost  its  supply  of 
sperm,  new  spermatozoa  begin  to  develop.  The  epithelial 
cells  lining  the  cavities  of  the  testes  divide  at  first  after  a 
type  of  cleavage  called,  by  Flemming,  homoeotypic.  This  first 
period  of  activity  produces  the  first  generation  of  spermato- 
cytes, which  divide  according  to  another  t3"pe,  the  heterotypic. 

The  cells  of  the  second  generation  of  spermatocytes  also  di- 
vide in  the  same  way,  but  with  an  occasional  homoeotypic  cleav- 
age. Finally,  in  the  third  generation  of  spermatocytes,  both 
types  of  cleavage  occur.  The  products  of  the  third  generation 
transform  directly  into  spermatozoa.  In  the  heterotypic  division 
the  process  is  as  follows.  The  chromatin  is  at  first  arranged  in 
a  thick  thread,  having  a  definite  arrangement.  The  skein-stage 
follows,  and  a  longitudinal  splitting  of  the  chromatin-thread  is 
apparent.  A  thickening  of  the  thread  then  takes  place,  and 
it  breaks  up  into  twelve  chromosomes  (only  half  the  number 
present  in  other  cells  of  the  body)  (Fig.  3,  A,  B).  At  the  free 
ends  of  the  bent  chromosomes,  each  of  which  is  split  longitudi- 
nally, the  halves  fuse  together  (Fig.  3,  B),  but  elsewhere  the 


1  Its  origin  in  the  frog  has  not  been  definitely  made  out.     It  is  probably 
cytoplasmic  in  origin  (Fig.  7,  A,  Bj  C). 


6 


DEVELOPMENT   OF   THE   FROG'S   EGG 


[Ch.  1 


halves  of  the  chromosomes  separate  from  each  other  along  the 
longitudinal  line  of  division.  The  process  is  similar  to  the  ring- 
formation  of  Gryllotalpa.  In  this  way  twelve  loops  are  formed 
from  the  twelve  chromosomes.  The  bent  ends  of  the  new  loops 
or  rings  correspond  to  the  middle  portions  of  the  earlier  rods  or 
chromosomes  (see  the  +  and  —  signs  in  Fig.  3,  A,  C).  Mean- 
while the  achromatic  spindle  between  the  centrosomes  has  devel- 
oped, and  the  loops  of  chromatin  are  arranged  on  the  threads  of 
the  spindle,  as  seen  in  Fig.  3,  B.     At  the  next  stage  each  loop 


B 


Fig.  3.  — Heterotypic  type  of  nuclear  division  in  Salamandra.     (After  Flemming.) 


breaks  at  the  equator,  i.e.  at  the  point  where  the  ends  of  the 
rods  fused  at  an  earlier  period,  and  begins  to  migrate  toward 
its  centrosome  (Fig  3,  D).  While  this  migration  of  the  twelve 
bent  chromosomes  is  taking  place,  each  chromosome  may  be 
seen  again  to  split  longitudinally,  although  the  two  halves 
remain  in  contact  (Fig.  3,  E).  The  cell  then  passes  into  a 
resting -stage. 

In  the  homoeotypic  division  the  first  phase,  the  spireme,  is  simi- 


Ch.  1] 


FORMATIOX  OF   THE   SEX-CELLS 


Lar  to  the  last,  i.e.  it  is  a  skein  (Fig.  4,  A)  with  longitudinally 
split  thread.  Twelve  bent  rods  appear  and  become  shorter  than 
the  bent  rods  of  the  heterotypic  type.  These  rods  then  arrange 
themselves  about  the  middle  of  the  achromatic  spindle  (Fig. 
4,  B).  The  twelve  bent  rods  divide  each  into  two  by  separa- 
tion along  a  longitudinal  line,  and  twenty-four  rods  are  present. 
Immediately  twelve  of  these  migrate  toward  one  pole,  and 
twelve  toward  the  other,  and  the  cell-division  follows  (Fig.  4, 
C,  D,  E,  F).     The  cells  then  come  to  rest. 


D  E  F 

Fig.  4.  —  Homceotypic  type  of  nuclear  division  in  Salamandra.     (After  Flemming.) 


The  end  result  in  the  two  types  of  cleavage  is  the  same,  but 
the  details  are,  as  described,  different.  It  is  important  to  note 
that  the  number  of  chromosomes  is  half  that  of  the  number  of 
chromosomes  in  the  other  cells  of  the  bod}'. 

Vom  Rath  maintains  that  a  fourth  generation  of  cells  appears 
in  the  development  of  the  spermatozoa  of  the  salamander.  Flem- 
ming supposed  that  at  the  end  of  the  third  generation  of  cells, 
described  above,  the  differentiation  into  spermatozoa  began,  but 


8 


DEVELOPMENT  OF  THE  FROG'S  EGG 


[Ch.  I 


vom  Rath  has  found  that  at  the  end  of  the  third  generation 
large  cells  appear  with  huge  nuclei  (Fig.  5,  B),  in  which  there 
are  twelve  groups  of  chromosomes.  Each  group  or  tetrad  is 
composed  of  four  granules.  There  are,  therefore,  present  forty- 
eight  spherical  chromosomes  united  in  groups  of  four.  These 
tetrads  arose  from  a  heterotypic  spindle,  and  in  the  following- 
way.  As  the  twelve  loops  (which  are  now  double,  making 
twenty-four  loops)  passed  toward  one  pole  they  became  much 
thicker  (Fig.  5,  A).  The  middle  point  of  union  of  each  of  the 
twenty-four  loops  broke  (Fig.  5,  B),  and  the  portions  rounded 
up,  so  that  there  were  present  forty-eight  chromosomes  arranged 
in  twelve  groups  of  four  chromosomes  each  (Fig.  5,  B).  Imme- 
diately after  the  formation  of  the  tetrads  the  groups  of  chromo- 
somes arranged  themselves  along  the  rays  of  the  achromatic 


Fig.  5.  — Formation  of  tetrads  in  testis  of  Salamandra.     (After  vom  Rath.) 


spindle  (Fig.  5,  C).  The  tetrads  next  passed  toward  the 
equator  of  the  spindle,  and  there  they  divided,  so  that  two  of 
each  of  the  four  chromosomes  passed  toward  one  pole  of  the  cell 
(as  in  Gryllotalpa).  In  this  way  two  new  cells  are  formed 
with  twenty-four  chromosomes  each.  A  second  division  suc- 
ceeds without  an  intervening  resting-stage,  and  the  number  of 
chromosomes  is  reduced,  so  that  each  cell  has  twelve  chromo- 
somes. The  cells  resulting  from  the  last  division,  having  each 
twelve  chromosomes,  differentiate  each  into  a  spermatozoon. 

The  second  division,  according  to  some  workers  (Boveri, 
Hertwig,  and  Brauer),  is  the  result  of  a  second  loyigitudinal 
division.  But  vom  Rath  holds  that  this  second  division  in  the 
Amphibia  and  in  Gryllotalpa  is  the  result  of  a  cross-division  of 


Ch.  I]  FORMATION   OF   THE   SEX-CELLS  9 

the  threads.  According  to  Boveri,  the  meaning  of  the  forma- 
tion of  the  tetrad  is  only  the  precocious  separation  of  the  chro- 
matin-threads  for  two  rapidly  succeeding  divisions  (without  an 
intermediate  resting- stage).  The  doubling  of  the  chromosomes 
previous  to  division  has,  he  thinks,  no  further  significance  than 
the  preparation  for  two  quickly  succeeding  divisions.  It  is  not 
obvious,  however,  in  the  development  of  the  spermatozoon  why 
this  rapid  division  should  take  place  at  this  time  and  at  no 
other  in  the  life  of  the  cell. 

Meves  ('96)  has  most  recently  reexamined  the  development 
of  the  spermatozoon  in  the  salamander.  His  results  differ  in 
several  respects  from  the  earlier  results  of  Flemming,  and  in 
one  essential  respect  from  the  work  of  vom  Rath.  According 
to  Meves,  the  germ-cells  undergo  many  divisions  in  the  upper 
part  of  the  testis.  The  chromatic  figure  is  that  of  the  usual 
type  of  division ;  and  tiventy-four  chromosomes  are  present.  As 
a  result  of  the  division,  the  cells  become  smaller,  and  each  cell 
becomes  surrounded  by  a  layer  of  connective  tissue.  Each  of 
these  cells  then  divides  many  times  according  to  the  usual  type 
of  division,  so  that  clusters  of  cells  are  produced  surrounded  by 
a  common  wall  of  connective  tissue.  Then  follows  the  resting- 
period,  in  which  the  cells  enlarge.  After  this  the  maturation- 
divisions  take  place.  Meves  thinks  that  most  probably  each 
cell  divides  only  twice  during  this  period,  as  in  other  forms. 
The  first  division  is  heterotypic,  and  now  for  the  first  time  the 
number  of  chromosomes  is  reduced  to  tivelve.  Without  a  resting- 
period  each  cell  again  divides,  the  twelve  chromosomes  splitting 
longitudinally.  This  second  division  is  homoeotypic.  Each 
cell,  containing  twelve  chromosomes,  then  transforms  directly 
into  a  spermatozoon. 

Meves  shows  therefore  that  Flemming  was  mistaken  in  regard 
to  the  number  of  cell-generations  that  are  present  in  the  sper- 
matogenesis of  the  seilamander,  and  further  that  Flemming 
failed  to  make  out  the  real  sequence  of  the  generations  and 
the  number  of  chromosomes  present  in  each.  More  important 
is  Meves'  statement  that,  iiorinally^  there  is  not  a  formation  of 
tetrads  as  vom  Rath  had  affirmed.  At  present  it  is  impossible 
to  decide  between  the  divergent  accounts  of  Meves  and  vom 


10 


DEVELOPMENT  OF  THE  FROG'S  EGG 


[Ch.I 


Rath,  and  we  must  suspend  judgment  until  further  Avork  throws 
more  light  upon  the  question. 

The  spermatogenesis  of  the  frog  has  not  been  worked  out  in 
the  same  detail  as  that  of  the  salamander,  yet  vom  Rath  ('95) 
has  made  certain  important  statements  in  regard  to  it.  Tlie 
prophase  of  the  mitoses,  before  the  ripening  period,  has  in  the 
frog  a  close  resemblance  to  the  skein-stage  of  the  heterotypic 
and  homoeotypic  variations  in  the  salamander.  But,  on  the 
other  hand,  in  the  metakinetic  stage  the  peculiarities  of  the 
homoeotypic  and  heterotypic  forms,  as  described  by  Flemming, 


B 


Fig.  6.  —  Stages  in  the  last  maturation-division  of  sperm-cells  of  Frog. 
(After  vom  Rath.)     (Figs.  C,  D,  F  slightly  modified.) 

are  absent.  The  formation  of  the  chromatin-rings  and  tetrad- 
groups  in  the  frog  (Fig.  6,  A,  B,  C)  differs  from  that  of  the  sala- 
mander and  is  much  more  like  that  of  Gryllotalpa.  The  rings, 
owing  to  the  strong  contraction  of  the  segments,  are  relatively 
small  in  the  frog,  but  proportionate^  thick  (Fig.  6,  B).  From 
each  ring  arise  the  four  spherical  chromosomes  of  each  tetrad- 
group  (Fig.  6,  C).  The  ring-stage  lasts  quite  long  in  Rana, 
judging  from  the  frequency  of  its  presence.  The  rings  lie  at 
the  periphery  of  the  nucleus. 


Ch.  I] 


FORMATION   OF   THE   SEX-CELLS 


11 


The  shape  of  the  spermatozoon  is  very  different  in  different 
species  of  frogs.  In  some  species  the  head  of  the  spermatozoon 
is  drawn  out  into  a  fine  point  (Fig.  7,  A,  E) ;  in  other  forms  it 
ends  bluntly  (Fig.  7,  D).  The  middle  piece  is  easily  found  in 
some  spermatozoa,  but  in  others  only  by  the  application  of 
special  reagents.     In  the  European  toad  (Fig.  7,  A)  the  tail  of 


B 


Fig.  7.  —  Spermatozoa.  A.  Bufo  cinereus.  B-C.  T^vo  stages  in  development  of  sper- 
matozoon ;  D.  Fully  formed  spermatozo(3n  of  Hyla  arborea.  E.  Spermatozoon 
of  Rana  esculenta  (the  tail  is  too  short).     (After  v.  la  Valette  St.  George.) 

the  spermatozoon  is  formed  by  a  flat  membrane  with  a  thick- 
ened border.  In  Hyla  arborea  and  Rana  esculenta  (Fig.  7,  D,  E) 
the  tail  of  the  spermatozoon  is  like  a  long  lash  or  thread.  In 
Rana  esculenta  the  head  measures  .01 5-. 021  mm.  in  length  and 
the  tail  .04  mm.  in  length. 


"Direct"  Division  of  the  Germ-cells 

In  the  testes  of  the  frog  and  of  other  Amphibia  are  often 
found   germ-cells   whose    nuclei   have  very  irregular   outlines. 


12 


DEVELOPMENT  OF  THE  FROG'S  EGG 


[Cii.  I 


Centrosomes  can  generally  be  demonstrated  in  these  resting-cells. 
Other  cells  have  the  nuclei  broken  up  into  a  number  of  smaller 
spheres.  Still  other  nuclei  may  have  a  deep  depression  on  one 
side,  as  though  the  nucleus  were  dividing  into  two  by  constric- 
tion. These  nuclei  have  often  been  described  as  dividing  by 
amitotic  or  direct  division,  i.e.  without  the  characteristic  mitotic 
division.  Meves  ('91)  has  stated  that  in  the  testes  of  the  sala- 
mander amitotic  division  occurs  regularly,  and  he  believes  that 
such  cells  will  later  form  spermatozoa.  Other  authors  (Bellonci 
and  vom  Rath),  admitting  that  such  a  division  may  take  place, 
affirm  that  such  cells  are  in  process  of  degeneration,  and  never 
subsequently  form  spermatozoa.  Vom  Rath  declares  that  cells 
that  have  once  divided  by  amitosis  can  never  again  divide  by 
karyokinesis  (mitosis),  and  that  such  cells  degenerate  later,  and 
do  not  ever  develop  into  sex-cells. 

Oogenesis 


$^^ 


■'f\^ 


The  origin  of  the  egg  in  the  ovary  of  the  frog  has  been 
studied  by  Schultze  ('87,  e),  but  many  important  details  are 

still  unknown.  The  egg 
derived  from  one  of  the 
cells  of  the  outer  layer  of 
the  ovary  is  surrounded 
by  a  large  number  of  fol- 
licle-cells. The  nucleus 
of  the  egg  consists  of  fine 
chromatic  threads,  of  a 
nuclear  sap,  and  of  scat- 
tered nucleoli.  As  the 
egg  enlarges  the  nucleus 
also  enlarges,  and  the 
chromatin  stains  more 
faintly  and  appears  in  the 
form  of  scattered  threads. 
The  nucleoli  stain  well 
and  become  larger  and  more  numerous  as  the  egg  enlarges, 
and  are  found  generally  around  the  periphery  of  the  nucleus. 
Certain   portions  of   the   protoplasm  now  begin   to   stain   dif- 


FiG.  8.  —  Ovai-iau  egg  of  Rana. 


Ch.  IJ  FORMATION   OF   THE   SEX-CELLS  13 

ferently  from  the  rest,  and  are  spoken  of  as  the  yolk-nuclei. 
They  seem,  in  some  way  not  fully  understood,  to  be  con- 
nected with  the  development  of  tlie  yolk-granules.^  The 
yolk-granules,  at  first  small  and  scattered,  grow  larger  and 
become  more  numerous  (Fig.  8).  Before  the  egg  leaves  the 
ovary,  the  nucleus  wanders  toward  the  periphery  and  places 
itself  under  the  black  pole  of  the  egg 
(Fig.  9).  When  the  surface  of  the 
egg  is  examined,  it  shows  a  lighter 
area,  owing  to  the  displacement  of  the 
pigment-granules  in  the  region  occu- 
pied by  the  nucleus.  The  nucleoli  at 
this  time  migrate  toward  the  centre  of 
the  nucleus,^  there  disintegrate,  and 
finally  disappear.  The  chromatin-mate- 
rial  draws  together  at  this  time  into  Fig.  9.  —  Section  of  ripe  ova- 
threads  which  stain  more  deeply,  the  "^°  ^^^*  ^'  ^'  ^^" 
nuclear  membrane  disappears,  an  achromatic  spindle  develops, 
and  the  egg  is  ready  to  extrude  the  first  polar  body. 

Comparison  of  Spermatogenesis  with  Oogenesis 

The  method  of  extrusion  of  the  polar  bodies  is  described  in 
the  next  chapter,  but  we  may  anticipate  this  account  in  order 
to  consider  here  a  remarkable  parallel  that  has  been  discovered 
between  the  formation  of  the  polar  bodies  and  the  formation  of 
the  spermatozoa.  In  the  latter,  as  we  have  seen,  two  successive 
divisions  follow  each  other  during  the  maturation-period  ivith- 
out  an  intervening  resthig-stage.  The  tetrad-groups  are  present 
at  the  beginning  of  the  process.  After  the  two  maturation- 
divisions  the  number  of  chromosomes  is  reduced  to  half  the 
number  characteristic  for  the  species.^  The  same  phenomena 
appear  when  the  polar  bodies  are  extruded  from  the  egg.  After 
the  extrusion  of  the  first  polar  body,  the  spindle  for  the  second 

^  Will  ('84)  describes  the  yolk-nuclei  as  arising  from  constrictions  of  the 
nucleus  set  free  with  their  nucleoli  into  the  protoplasm. 

2  Schultze  affirms  that  the  later  chromatin  comes  from  these  nucleoli,  hut 
Born  has  corrected  this  statement. 

3  The  reduction  in  number  of  the  chromosomes  seems  in  some  forms  to  take 
place  before  the  tetrad-period. 


14  DEVELOPMENT   OF   THE  .FROG'S   EGG  [Ch.  I 

polar  body  forms  immediately  without  a  resting-period.  Further, 
it  is  found,  after  the  extrusion  of  the  second  polar  body,  that  the 
number  of  chromosomes  in  the  egg  is  reduced  to  half  the  num- 
ber characteristic  for  the  somatic  cells.  Tetrads  have  also  been 
described  as  occurring  just  before  the  extrusion  of  the  polar 
bodies.  In  many  cases  the  first  polar  body  divides  into  two, 
so  that  three  polar  bodies  are  present.  These  three  polar  bodies 
and  the  Qgg  seem  to  correspond  to  the  four  spermatozoa  from 
each  spermatocyte.  All  four  spermatozoa  are  functional,  but 
only  the  Qgg  (and  not  its  three  polar  bodies)  is  capable  of  devel- 
opment. Weismann  has  utilized  the  discovery  of  the  reduction  of 
the  number  of  chromosomes  to  build  up  an  elaborate  and  highly 
speculative  theory  of  heredity.  The  reduction  division  is,  ac- 
cording to  Weismann,  not  simply  a  quantitative  division  of  the 
chromatic  thread,  but  is  at  one  stage  at  least  a  qualitative 
division.  The  reduction  of  the  chromosomes  to  half  the  number 
present  in  the  other  cells  of  the  body  seems,  according  to  Weis- 
mann and  others,  to  be  a  preparation  for  fertilization.  Since 
the  spermatozoon  brings  into  the  egg  only  half  the  number  of 
chromosomes  found  in  the  somatic  cells  of  the  animal,  and 
since  the  egg-nucleus  supplies  the  other  half,  the  number  of 
chromosomes  will  thus  remain  constant  for  the  species  from 
generation  to  generation. 


CHAPTER   II 
POLAR  BODIES  AND   FERTILIZATION 

Whether  the  egg  leaves  the  ovary  by  means  of  its  own 
activity,  or  by  some  other  mechanism,  we  do  not  know.  That 
the  egg  itself  takes  some  part  in  the  process  seems  possible 
from  the  fact  that  it  is  set  free  only  at  a  particular  moment 
of  its  maturation,  i.e.  at  a  time  when  certain  processes  have 
taken  place  in  its  interior.  This  same  process  takes  place 
simultaneously  in  all  the  eggs  in  the  ovary.  The  separation  of 
the  egg  from  the  ovary  is  not  dependent  upon  the  act  of  copu- 
lation, for  several  cases  are  on  record  in  which  isolated  females 
were  found  to  have  eggs  in  the  body-cavity  and  oviducts. 

The  egg  set  free  in  the  ccelomic  cavity  is  covered  by  an  ex- 
tremely thin  membrane,  the  egg-membrane  or  vitelline  mem- 
brane. The  egg  itself  is  very  soft  and  easily  broken  if  handled. 
Later,  when  in  the  oviduct,  the  protoplasm  seems  to  become 
more  firm. 

The  egg  shows  a  white  and  a  dark  hemisphere.  The  relative 
distribution  of  superficial  pigment  in  the  egg  determines  the 
extent  of  the  white  and  dark  surfaces.  The  outer  layer  of  pig- 
ment in  the  black  hemisphere  seems  to  be  in  close  contact  with, 
or  fixed  to,  the  vitelline  membrane,  but  the  pigment  lying  in 
the  protoplasm  beneath  the  outer  layer  is  free  to  move  with  any 
movement  of  the  protoplasm  (Figs.  8,  9).  The  relative  ex- 
tent of  surface  of  the  egg  that  is  black  or  white  is  variable  in 
different  species,  and  even  in  different  females  of  the  same 
species ;  but  all  the  eggs  from  one  female  show  approximately 
the  same  distribution  of  pigment. 

Extrusion  of  the  First  Polar  Body  axd  Egg-layixg 

Just  prior  to  the  extrusion  of  the  egg  into  the  body-cavity  of 
the  frog,  the  nucleus  undergoes  a  remarkable  change,  so  that  in 

15 


16  DEVELOPMENT   OF   THE   FROG'S   EGG  [Cii.  II 

place  of  a  large  watery  nucleus  only  a  small  mass  of  chromatic 
substance,  lying  in  the  protoplasm,  is  present.  An  achromatic 
spindle  appears,  and  the  chromatin  in  the  form  of  granules  is 
arranged  at  the  equator  of  the  spindle.  The  spindle  lies  at  the 
surface  of  the  egg  near  the  centre  of  the  black  hemisphere 
(Fig.  11,  A).  It  lies  also  in  the  centre  of  the  fovea,  which  is 
found  on  the  surface  of  the  egg.  The  fovea  marks  the  former 
position  of  the  large  ovarian  nucleus,  and  although  the  nuclear 
membrane  of  the  original  nucleus  has  disappeared,  and  its 
watery  cavity  has  been  encroached  upon  by  the  surrounding 
protoplasm,  yet  the  pigment  has  not  penetrated  very  deeply 
into  this  region.  The  eggs  pass  in  this  condition  from  the 
body-cavity  into  the  oviducts.  Newport  ('51)  believed  that, 
owing  to  the  close  attachment  of  the  oviducts  at  their  inner 
openings  to  the  walls  of  the  pericardium,  at  each  contraction  of 
the  heart  the  slit-like  openings  of  the  oviducts  would  gape  open, 
and  any  eggs  in  the  vicinity  might  be  forced  into  the  mouths 
of  the  tubes.  Also,  he  thought  that  owing  to  the  muscu- 
lar movements  of  the  body,  and  the  resulting  shifting  of  the 
internal  organs,  the  eggs  sooner  or  later  pass  near  the  openings 
of  the  oviduct,  and  are  then  carried  into  the  tube.  At  any 
rate,  there  seems  to  be  not  much  ground  for  the  older  state- 
ment that  the  mouths  of  the  oviducts  actually  grasp  the  eggs 
by  a  muscular  movement  like  that  of  swallowing.  According 
to  Nussbaum  ('95),  the  eggs,  when  set  free  from  the  ovary  into 
the  body-cavity  of  the  frog,  are  carried  into  the  open  mouths 
of  the  oviducts  by  the  motion  of  the  cilia  of  the  coelomic  epi- 
thelium. These  cilia  drive  anteriorly  any  bodies  lying  free 
in  the  body-cavity.  If,  for  instance,  eggs  taken  from  one  frog 
be  placed  in  the  vicinity  of  the  openings  of  the  oviducts  in 
the  body-cavity  of  another  frog,  they  will  be  carried  into  the 
open  mouths  of  the  oviducts  by  the  action  of  the  cilia  in  that 
region. 

The  cilia  do  not  cover  the  entire  surface  of  the  coelomic 
epithelium,  and  there  are  certain  recesses  in  the  body-cavity 
destitute  of  cilia.  The  eggs  that  accumulate  in  these  recesses 
will  be  sooner  or  later  forced  out  into  the  general  cavity  as  a 
result  of  the  alternate  contractions  and  expansions  of  the  ven- 
tral musculature  of  the  body-wall,  as  well  as  by  the  changes 


Ch.  II] 


POLAR   BODIES   AND   FERTILIZATION 


17 


produced  by  the  filling  and  emptying  of  the  lungs,  and  by  the 
movements  of  the  heart. 

Swammerdam's  account  in  1737  describes  the  passage  of  the 
egg  from  the  ovaries  to  the  oviducts  b}^  way  of  the  coelomic 
space.  Spallanzani  in  1785  observed  that  the  females  of  Bufo 
igneus,  isolated  before  union  with  the  male,  could  still  lay  their 
eggs.  One  of  the  tree-frogs  has  its  eggs  in  the  uterus  before 
it  unites  with  the  male.  On  the  other  hand,  Spallanzani  stated 
that  females  of  the  stinking  toad  if  isolated  while  the  eggs  are 
still  in  the  ovaries  will  retain  their  eggs,  but  if  separated  after 
having  paired  will  then  deposit  their  eggs.  According  to  the 
evidence  of  several  authors,  Rana  temporaria  when  isolated  will, 
in  certain  cases  at  least,  set  free  its  eggs. 

It  has  been  suggested  that  the  embrace  of  the  male  is  me- 
chanically necessary  in  order  that  the  eggs  may  pass  from  the 
ovary  into  the  oviducts, 

but  this  is  certainly  not  ^^    ^      ^'"~'~~^'^^-^. 

always  the  case,  and  if 
not  necessary  in  one  form 
is  probably  not  necessary 
in  others.  The  sexual 
excitement  set  up  by  the 
tight  embrace  of  the  male 
may  however  be  neces- 
sary in  some  species  for 
the  successful  perform- 
ance of  egg-laying.  The 
eggs  pass  one  by  one 
down  the  length  of  the 
oviducts,  ultimately  to 
reach  the  lower  portion 

of  the  tube,  the  so-called  uterus,  where  the  eggs  accumulate. 
If  a  frog  is  killed  at  the  height  of  the  breeding  season,  free 
eggs  are  often  found  in  the  body-cavity,  and  a  series  of  eggs 
passing  individually  down  the  ovarian  tubes,  as  well  as  an 
accumulation  of  eggs  in  the  uteri.  In  their  passage  through 
the  oviducts  the  eggs  undergo  certain  internal  changes  and  re- 
ceive also  their  egg-coats.  In  the  tubes  of  the  oviducts  the  nu- 
clear spindle  divides,  so  that  half  of  the  original  chromatin  goes 


Fig.  10.  —  Egg  in  jeUy.     (After  Schultze.) 


18 


DEVELOPMENT  OF  THE  FROG'S  EGG 


[Cii.  II 


to  one  pole  of  the  spindle,  and  half  to  the  other.  The  spmdle 
has  assumed,  during  this  time,  a  radial  position  with  respect  to 
the  egg,  so  that  we  may  speak  of  a  distal  and  of  a  proximal  or 
central  end  (Fig.  11,  A).  The  distal  end  pushes  out  into  a 
protrusion  of  protoplasm  that  has  simultaneously  formed  at  this 


c=>d 


A 


1^. 


B 


""■Wfu 


..^a 


0?q^o^^Sc«° 


oO'ii 


,,j^sM}^&o 


""^^Op. 


cy 


Fig.  11.  —  Extrusion  of  first  polar  body  and  fertilization  of  egg  of  Toad.  (From 
preparations  made  by  Helen  D.  King.)  A.  First  polar  spindle.  B.  First  polar 
body  extruded;  second  polar  spindle  present.  C.  Entrance  of  spermatozoon. 
D.  Male  and  female  pronuclei.  E.  Apposition  of  two  pronuclei.  F.  First  seg- 
mentation-spindle. 


point  of  the  surface  of  the  egg.  This  protrusion  of  protoplasm 
with  its  enclosed  half  of  the  nucleus  gradually  pinches  off  from 
the  surface  of  the  egg,  and  there  is  thus  formed  the  first  polar 
body  (Fig.  11,  B).     The  egg  gets  a  thin  layer  of  gelatinous 


Ch.  II]  POLAR  BODIES   AXD   FERTILIZATION  19 

substance  around  it  soon  after  entering  the  oviduct,  i.e,  before 
it  has  reached  the  first  part  of  the  convoluted  portion.  This  is  , 
the  so-called  chorion,  —  a  thin  investing  membrane  which  ad- 
heres closely  to  the  vitelline  layer  around  the  egg.  During  the 
remainder  of  the  passage  through  the  oviducal  tube  the  Qgg  gets 
two  other  distinct  gelatinous  layers  (Fig.  10).  The  middle 
layer  of  the  three  is,  according  to  Newport,  a  watery  layer  of 
considerable  thickness.  The  outer  gelatinous  covering  is  also 
thick  and  serves  to  stick  the  eggs  together  in  a  bunch,  and  even 
to  stick  the  bunches  of  eggs,  when  laid,  to  surrounding  objects. 

The  spawning  of  certain  species  of  frogs  takes  place  very  rap- 
idly, and  by  a  single  effort.  Newport  says  that  the  process 
takes  place  in  a  few  seconds  or  less  than  a  minute,  and  that  all 
the  eggs  that  have  accumulated  in  the  uteri  are  laid  at  once. 
When  laid,  the  egg-cluster  forms  a  rounded  mass  which  is,  at 
first,  scarcely  as  large  as  a  walnut.  The  eggs  then  seem  to  con- 
sist almost  entirely  of  dark-colored  "yelks"  with  thin  gelatinous 
envelopes.  "  Up  to  about  this  period  the  ova  remain  undisturbed 
in  the  water  in  a  mass  as  they  are  expelled,  and  lie  indiscrimi- 
nately, some  with  the  dark  and  some  with  the  white  portion  of 
the  yelk  uppermost  or  horizontal.  But  during  the  time  that 
has  passed  since  the  ova  have  been  in  contact  with  the  water, 
the  envelopes  have  imbibed  fluid  and  expanded  until  these  in- 
vestments of  the  yelk  have  a  thickness  equal  to  about  two- 
thirds  of  the  diameter  of  the  yelk  itself." 

"The  yelks,  that  have  remained  up  to  this  time  with  their 
white  surface  uppermost,  now  change  their  position  spontane- 
ously by  a  partial  rotation  of  the  whole  mass  of  each  on  its 
axis,  within  the  vitelline  membrane,  until  the  dark  surface  of 
the  whole  is  placed  uppermost.  Whether  this  change  of  posi- 
tion is  merely  the  result  of  expansion  of  the  vitelline  membrane 
at  this  period,  or  whether  it  be  also  connected,  as  we  may 
fairly  believe,  with  changes  going  on  in  the  interior  of  the 
yelk,  I  am  not  prepared  to  decide." 

The  Jelly  of  the  Egg,  and  the  Second  Polar  Body 

The  jelly  around  the  frog's  Qgg  serves,  no  doubt,  as  a  pro- 
tection to  the  Qgg.     The  soft  eggs  are  kept  in  spherical  shape 


20        DEVELOPMENT  OF  THE  FROG'S  EGG     [Ch.  II 

and  protected  from  injury  from  without.  The  slime  protects 
them  from  water-snails  that  will  eat  the  eggs  if  they  are  shelled 
out  from  the  jelly.  The  jelly  may  also  protect  them  against 
water-birds.  The  eggs  and  young  tadpoles  seem,  however,  in 
themselves  to  be  distasteful  to  certain  Crustacea  (Bernard  and 
Bratuschek,  '91). 

This  jelly  has  the  physical  peculiarity  of  allowing  the  sun's 
rays  to  pass  through,  but  hinders  reflection  of  the  rays  from 
the  interior  to  the  outside.  The  result  is  that  in  the  sunlight 
the  mass  of  eggs  is  at  a  higher  temperature  than  the  surround- 
ing water,  and  as  the  eggs  of  many  frogs  are  laid  in  the  early 
spring,  when  the  water  is  quite  cool,  this  property  of  the  jelly 
helps  to  hasten  their  development. 

Hertwig  ('T7)  thought  that  a  change  takes  place  in  the  inte- 
rior of  the  egg  after  fertilization,  so  that  a  difference  in  the 
specific  gravity  of  different  parts  of  the  egg  is  brought  about. 
Schultze  ('87),  however,  pointed  out  that  at  this  period  the 
egg  contracts  slightly  from  its  vitelline  membrane,  and  between 
the  egg  and  its  membrane  a  fluid  collects,  that  is  probably 
squeezed  out  of  the  egg  itself.  The  egg^  freed  from  its  inner- 
most coat  which  held  it  in  place,  then  rapidly  orients  itself  with 
respect  to  gravity.  Unfertilized  eggs  will  also,  after  a  time, 
slowly  rotate,  and  in  these  it  can  be  seen  that  the  separation 
of  the  egg  from  its  membrane  is  less  perfect  than  in  fertilized 
eggs.  "At  the  moment  Avhen  the  ovum  is  expelled  from  the 
body,  the  envelope  is  merely  a  thin  gelatinous  layer,  its  entire 
diameter  being  equal  only  to  about  one-sixth  of  the  diameter 
of  the  yelk.  After  it  has  been  one  ininute  in  water,  and  begun 
to  imbibe  and  expand,  it  is  then  equal  to  about  one-fourth  of 
the  diameter  of  the  yelk.  At  the  end  of  two  minutes  it  is  en- 
larged to  one-third,  and  in  three  minutes^  to  one-half  the  diame- 
ter of  this  body.  In  four  minutes,  it  exceeds  three-fifths,  and 
in  six  minutes,  two-thirds,  and  it  continues  to  imbibe  fluid  and 
expand  at  the  same  rate,  until,  at  from  ten  to  fifteen  minutes, 
it  very  nearly  equals  in  thickness  the  whole  diameter  of  the 
yelk ;  and  at  half  an  hour  it  is  one-fourth  greater  than  this. 
At  the  end  of  three  hours  the  membranes  have  acquired  nearly 
their  full  size." 

"The  expansion  of  the  envelope  is  greatly  retarded  at  the 


Ch.  II]  POLAR  BODIES   AND   FERTILIZATION  21 

end  of  the  third  or  fourth  hour,  until  after  cleavage  of  the  yelk 
has  taken  place,  when  it  again  proceeds,  but  much  more  slowly 
than  at  first." ^ 

In  Rana  fusca  the  extrusion  of  the  second  polar  body  takes 
place  one  half-hour  after  fertilization,  and  the  process  can  be 
seen  under  a  low  magnifying  glass  or  even  with  the  naked 
eye.  A  whitish  speck  appears  in  the  black  hemisphere  near 
the  point  at  which  the  first  polar  body  was  extruded.  It  is 
necessary,  however,  to  make  sections  of  the  Q^g  to  discover  the 
further  changes  that  are  taking  place.  Schultze  ('87)  has 
given  a  careful  description  of  the  process.  The  nucleus  that 
remains  in  the  Qgg  after  the  extrusion  of  the  first  polar  body 
assumes  once  more  a  horizontal  position,  but  does  not  go  irito  a 
resting-stage  (Fig.  11,  B),  i.e.  the  chromatic  loops  or  threads 
do  not  re-fuse  into  a  network  nor  does  a  nuclear  membrane 
form.  The  chromatin  arranges  itself  on  a  new  spindle.  The 
latter  then  assumes  a  more  or  less  radial  position,  and  the 
second  polar  body  is  extruded  half  an  hour  after  the  egg  is 
laid.  It  is  probable  that  the  second  polar  body  is  not  ex- 
truded under  normal  conditions  until  after  a  spermatozoon  has 
entered  the  Qgg. 

One  and  a  half  hours  after  the  Qgg  is  laid,  another  change 
may  be  seen  taking  place.  Near  to  or  at  the  apex  of  the  black 
pole  the  Qgg  is  seen  to  flatten,  and  an  accumulation  of  fluid 
is  found  here  between  the  Qgg  and  its  vitelline  membrane 
(Fig.  10).  At  or  near  the  centre  of  this  flattened  portion  one 
may  see  the  fovea,  and  near  or  in  it  the  polar  bodies  appear  on 
the  flattened  disc.  This  chamber  formed  between  the  flattened 
Qgg  and  the  inner  membrane  was  seen  by  Newport  and  called 
the  "respiratory  chamber."  It  may  ultimately  be  as  large  as 
one-sixth  the  diameter  of  the  whole  Qgg.  Schultze  points  out 
that  it  lies  somewhat  excentrically  with  respect  to  the  egg-axis 
(Fig.  10).  The  clear  fluid  in  this  chamber  has  been  supposed 
to  be  the  watery  contents  of  the  original  large  nucleus  of  the 
Qgg^  which  has  been  squeezed  out  of  the  egg.  Very  little 
evidence  has  as  yet  been  given  to  support  this  view.  Some 
of  the  older  embryologists  thought  that  this  fluid  represented 

1  Newport  ('51),  p.  193. 


22        DEVELOPMENT  OF  THE  FROG'S  EGG     [Ch.  II 

the  original  egg-nucleus  itself,  which  was  squeezed  out  of  the 
egg  at  this  time.  Now,  however,  since  we  know  the  complete 
history  of  the  nucleus  during  this  period,  the  suggestion  of  its 
entire  loss  by  the  egg  does  not  call  for  serious  criticism. 

Entrance  of  Spermatozoon  and  Copulation  of 
Pronuclei 

The  sperm  of  the  male  is  poured  out  into  the  water,  and 
probably  over  the  eggs  themselves  at  the  moment  when  they 
are  laid,  and  the  spermatozoa  begin  at  once  to  bore  into  the 
jelly  of  the  egg-mass  (Fig.  10). 

Kupffer  has  described  the  entrance  of  the  spermatozoon  into 
the  eggs  of  Bufo  variabilis.  When  the  head  of  a  spermatozoon 
touches  the  egg-membrane,  the  protoplasm  of  the  egg  draws 
back  slightly  at  the  point  of  contact,  but  quickly  returns  again 
to  its  first  position.  The  period  of  penetration  of  the  sperma- 
tozoon from  the  moment  of  contact  of  the  sperm-head  until  the 
spermatozoon  disappears  into  the  egg^  lasts  in  some  cases  from 
one  to  one  and  a  half  minutes,  in  other  cases  only  three-fourths 
of  a  minute.  Several  spermatozoa  were  observed  by  Kupffer 
to  enter  each  egg. 

Other  spermatozoa  reach  the  egg-membrane,  but  do  not  seem 
to  be  able  to  enter  the  egg.  In  the  regions  where  these  sper- 
matozoa lie,  the  surface  of  the  egg  rises  up  in  small  protuber- 
ances. This  process  occurs  about  fifteen  minutes  after  the  first 
spermatozoa  have  entered,  and  lasts  about  one  or  two  minutes, 
after  which  the  protuberances  sink  back  into  the  egg.  The 
spermatozoa  in  the  regions  of  the  protuberances  are  left  outside 
the  egg-membrane.  This  peculiar  phenomenon  is  described  by 
Kupffer  as  a  counter  demonstration  of  the  egg  against  those 
spermatozoa  that  have  not  been  able  to  enter.  Eggs  that  have 
been  artificially  fertilized  show,  when  cut  into  sections,  that  one 
hour  after  fertilization  a  dark  pigmented  streak  is  formed, 
reaching  from  the  pigmented  coating  of  the  egg  into  the  yolk- 
mass.  The  process  takes  place  in  the  upper  or  dark  hemi- 
sphere, and  regularly  at  one  side  of  the  centre  of  the  dark  field 
near  to  the  edge  of  the  white  border.  The  streak  takes  a 
somewhat  oblique  course  toward  the  centre  of   the  egg.     At 


Ch.  II]  POLAR   BODIES   AND   FERTILIZATION  23 

the  central  end  the  dark  streak  is  rounded,  and  encloses  a  clear 
spot. 

In  this  clear  region  one  sees  a  distinct  pronucleus  about  nine 
microns  (/x)  in  diameter.  Eggs  one  and  a  half  hours  after 
fertilization  show  that  the  pigmented  streak  has  penetrated 
deeper  into  the  egg^  and  in  the  frog  the  male  pronucleus  has 
enlarged  to  32  by  22  /x  (Fig.  11,  D,  for  the  toad). 

At  this  stage  another  nucleus  is  present  in  the  frog's  egg^ 
and  this  lies  not  far  from  the  end  of  the  pigmented  streak 
(Fig.  11,  D).  This  measures  22  /z,  and  has  the  same  structure 
as  the  male  nucleus.  These  two  nuclei  are  undoubtedly  the 
male  and  female  pronuclei.  We  now  know  that  the  female 
pronucleus  has  come  directly  from  the  original  egg-nucleus, 
which  has,  after  extruding  its  two  polar  bodies,  penetrated 
once  more  deeper  into  the  egg.  The  complete  history  has  not 
been  traced  in  the  frog,  but  there  can  be  no  reasonable  doubt 
as  to  what  takes  place.  In  the  newt  (and  in  the  toad)  the 
history  has  been  followed,  and  it  is  found  that  the  female  pro- 
nucleus arises  from  the  egg-nucleus  after  the  extrusion  of  the 
polar  bodies. 

In  the  next  half -hour  Hertwig  has  found  that  the  nuclei 
approach  more  nearly  to  each  other,  and  the  pigment-streak 
penetrates  deeper  into  the  egg^  the  swollen  end  enlarges, 
and  the  two  large  oval  male  and  female  pronuclei  are  then 
found  together  in  the  swollen  end  of  the  streak  (Fig.  11,  E). 
In  a  preparation  of  an  older  stage  both  nuclei  have  increased 
in  volume  to  35  /a,  and  have  flattened  against  each  other.  Thei/ 
then  fuse  into  one  nucleus  which  measures  44  /i  (Fig.  11,  F,  toad). 
The  resulting  nucleus,  the  segmentation-nucleus,  is  surrounded 
by  clear  protoplasm  and  then  by  a  pigment-coat.  From  the 
segmentation-nucleus  a  streak  of  pigment  extends  to  the  dark 
surface  of  the  egg  and  marks  the  path  of  entrance  of  the 
spermatozoon.  All  preparations  after  two  and  a  half  hours 
showed  the  union  of  the  two  pronuclei. 

If  the  jelly  be  examined  after  the  eggs  have  been  laid, 
several  or  many  spermatozoa  can  be  seen  boring  their  way 
through  the  jelly  toward  the  egg.  Some  will  have  reached 
the  inner  layers,  and  still  others  lie  in  the  outer  coats  (Fig. 
10).     It  is  probable  that  after  one  spermatozoon  has  succeeded 


24        DEVELOPMENT  OF  THE  FROG'S  EGG     [Ch.  II 

in  forcing  its  way  through  the  inner  coat  and  into  the  Qgg^ 
changes  then  take  place  in  the  egg  that  prevent  or  make 
difficult  the  further  entrance  of  other  spermatozoa.  The  con- 
traction of  the  Qgg^  noted  above,  may  possibly  have  something 
to  do  with  the  process.  If,  however,  two  or  more  sper- 
matozoa should  reach  the  surface  of  tlie  Qgg  at  about  the 
same  moment,  it  is  not  improbable  that  more  than  one  might 
enter.  1  Both  may  then  pass  toward  the  female  pronucleus,  but 
in  the  frog  it  is  probable  that  after  one  male  pronucleus  has 
fused  with  the  female  pronucleus,  the  further  progress  of  other 
male  pronuclei  that  happen  to  get  into  the  Qgg  is  stopped. 

It  is  sometimes  said  that  the  female  pronucleus  attracts  the 
male  pronucleus,  but  the  approach  of  the  two  may  be  due  to 
changes  in  the  protoplasm ;  for  the  migration  of  the  pronuclei 
through  the  egg  is  probably  in  most  cases  brought  about  by 
the  protoplasm  of  the  Qgg  under  the  influence  of  the  pronuclei, 
and  the  pronuclei  themselves  are  merel}^  passively  carried 
along. 

In  the  newt  (Jordan,  '93)  it  seems  to  be  usual  for  more  than 
one  spermatozoon  to  enter,  but  only  one  of  these  fuses  Avith  the 
female  pronucleus.  The  others  subsequently  degenerate  and 
go  to  pieces.  In  the  eggs  of  other  animals,  as  the  starfish, 
polyspermy,  or  the  entrance  of  more  than  one  spermatozoon 
into  the  Qgg^  brings  about  disastrous  results,  causing  irregular 
division  of  the  nucleus  and  subsequent  irregularities  in  the 
segmentation  of  the  Qgg.  In  these  eggs  the  field  of  action  is 
small,  and  the  male  pronuclei  or  their  centrosomes  mutually 
influence  one  another  and  the  female  pronucleus.  In  large 
eggs  with  much  yolk,  such  as  those  of  the  Amphibia  and  of  the 
Sauropsida,  the  spermatozoa  may  be  too  far  apart  to  affect  one 
another  or  the  segmentation-nucleus,  and  after  the  fusion  of 
one  male  pronucleus  with  the  female  the  movement  of  the 
other  male  pronuclei  towards  the  female  pronucleus  seems  to 
stop. 

The  head  of  the  spermatozoon  enters  the  Qgg  to  become  the 
male  pronucleus.     The  tail  of  the  spermatozoon  is  left  at  the 


lit  is  prolDable  that  Kupffer's  ('82)  account  does  not  apply  to  eggs  under 
normal  conditions. 


Ch.  II]  POLAR  BODIES  AND   FERTILIZATION  25 

surface  of  the  egg,  or  if  a  part  enter  the  egg  it  takes  no  share 
in  the  subsequent  changes.  The  middle  piece  of  the  sperma- 
tozoon is  now  known  to  contain  a  body  that  plays  a  most  con- 
spicuous part  in  many  animals  in  the  division  or  cleavage  of 
the  egg.  The  middle  piece  enters  with  the  head  of  the  sper- 
matozoon. It  contains  the  centrosome,  which  divides,  and 
around  each  centre  an  elaborate  system  of  rays  develops.  The 
two  centrosomes  migrate  to  opposite  sides  of  the  segmen- 
tation-nucleus, and  between  the  two  appears  the  spindle  of 
the  first  cleavage.  In  the  frog  the  history  of  the  middle 
piece  and  centrosome,  and  the  origin  of  the  segmentation- 
spindle  have  not  yet  been  worked  out. 


CHAPTER   III 

EXPERIMENTS  IN  CROSS-FERTILIZATION 

A  NUMBER  of  attempts  have  been  made  to  fertilize  the  eggs 
of  one  species  of  frog  with  the  spermatozoa  of  another  species. 
Rusconi  experimented  in  1840  with  the  toad  (  $  )  and  the  green 
water-frog,  Rana  esculenta  (  9  ).  Lataste  in  1878  attempted  to 
cross-fertilize  the  eggs  of  different  species  of  urodeles  with 
Pelobates  fuscus  and  P.  cultripes. 

Experiments  of  Pfluger  and  of  Born 

The  most  extensive  and  important  work  is  that  of  Pfliiger  ('82) 
and  of  Born  ('88).  These  investigators  have  made  a  large 
number  of  experiments  in  crossing  different  races  and  species 
of  Anura.  When  the  sperm  of  Rana  fusca  was  placed  with 
the  eggs  of  Buf o  vulgaris,  the  eggs  segmented  and  developed 
as  far  as  the  ''  morula  "  stage,  and  then  without  exception  died.^ 
Conversely,  when  the  sperm  from  B.  vulgaris  was  used  with  the 
eggs  of  R.  fusca,  no  result  followed,  not  even  the  segmentation 
of  the  egg  (except  in  one  experiment  where  two  eggs  out  of  one 
hundred  divided  irregularly).  Eggs  of  R.  fusca  placed  in  a 
water-extract  of  the  testes  of  R.  esculenta  remained  unfertil- 
ized. But  eggs  of  R.  esculenta  placed  with  the  sperm  of  R. 
fusca  developed  regularly,  with  few  exceptions,  as  far  as  the 
blastula  stage,  and  then  died.  Crossing  various  species  of  Tri- 
tons gave  no  results.  But  eggs  of  Rana  fusca  were  acted  upon 
by  the  sperm  of  Triton  alpestris  and  Triton  tseniatus,  inasmuch 
as  they  began  to  show  irregular  cleavage-lines.  Later  they 
died.     The  reverse  cross  gave  no  result. 

Rana  fusca  and  Rana  arvalis  are  very  similar  in  appearance, 
but   are   apparently  separate  species.     Cross-fertilization  was 

1  Pfliiger  ('82). 
26 


Ch.  Ill]       EXPERIMENTS   IN   CROSS-FERTILIZATION  27 

here  possible  (R.  fusca,  $  ,  R.  arvalis,  9  ).  Tadpoles  developed 
from  the  crossed  eggs,  and  some  of  these  ultimately  transformed 
into  frogs.  Pfliiger  got  similar  results  with  the  same  species,  and 
also  found  that  the  reverse  cross  (R.  fusca,  $  ,  and  R.  arvalis,  $  ) 
gave  no  result.  Born  found  that  the  eggs  of  Bufo  cinereus 
could  readily  be  fertilized  with  the  sperm  of  Bufo  variabilis. 
All  the  eggs  segmented  regularly,  the  larvae  left  the  jelly,  and 
developed  into  frogs. 

In  respect  to  the  closeness  of  the  relation  between  the  species, 
Born  says  that  we  can  be  quite  certain  that  the  two  species  of 
Rana  arvalis  and  R.  fusca  are  much  more  nearly  related  than  the 
two  species  of  Bufo.  The  success  of  cross-fertilizing  depends 
apparently  less  on  the  degree  of  relationship,  as  shown  by  the 
similarity  of  color  and  habits,  than  on  the  similarity  of  the  male 
sexual  products  (Pfliiger).  Although  R.  fusca  and  R.  arvalis 
seem  to  be  very  closely  allied  species,  they  have  very  different 
spermatozoa ;  in  fact,  the  spermatozoa  are  as  different  as  the 
spermatozoa  of  R.  fusca  and  R.  esculenta.^  The  two  species  of 
toads  (Bufo)  have  very  similar  spermatozoa,  which  differ  only 
in  size,  but  this  difference  is  so  slight  that,  were  the  two  kinds 
mixed  together,  one  could  scarcely  distinguish  between  them. 
It  is  apparently  owing  to  the  difference  in  form  of  the  sperma- 
tozoa of  the  R.  fusca  and  R.  arvalis,  and  to  the  similarity  of  the 
spermatozoa  of  B.  cinereus  and  B.  variabilis  that  the  results  are 
due. 

Pfliiger  has  made  a  large  number  of  reciprocal  crosses  between 
different  races  of  R.  fusca.  "  The  different  races  are  as  fertile 
inter  se  as  are  individuals  of  the  same  race."  Pfliiger  concluded, 
after  comparing  the  results  of  all  of  his  experiments  on  cross- 
fertilization,  that  in  general  those  spermatozoa  are  most  successful 
for  purposes  of  cross-fertilization  that  have  the  thinnest  and  most 
pointed  heads.  That  in  general  those  eggs  are  most  easily  fer- 
tilized that  belong  to  species  having  spermatozoa  with  thick 
heads.  The  results,  then,  he  thought,  depend  largely  upon  me- 
chanical conditions ;  for  where  the  head  is  small  and  pointed,  the 
spermatozoon  can  bore  its  way  more  successfully  into  the  eggs 


1  R.  arvalis  and  R.  esculenta  have  similar  sperm.     Born  and  Pfliiger  found 
that  the  crossed  eggs  segmented  irregularly,  and  that  later  the  embryos  all  died. 


28  DEVELOPMENT  OF  THE   FROG'S  EGG  [Ch.  Ill 

of  its  own  and  of  other  species.  If  the  head  is  large,  the  sper- 
matozoon can  force  its  way  only  into  those  eggs  that  are  adapted 
to  spermatozoa  with  large  heads.  For  instance,  the  sperma- 
tozoa of  R.  fusca  have  thinner  heads  than  any  others,  and 
the  head  is,  moreover,  very  pointed.  These  spermatozoa  can 
fertilize  eggs  of  nearly  all  other  species  (R.  arvalis,  R.  escu- 
lenta,  B.  communis).  Conversely,  the  thick-headed  spermatozoa 
of  R.  arvalis  and  the  blunt-headed  spermatozoa  of  R.  esculenta 
cannot  get  into  the  eggs  of  R.  fusca. 

The  spermatozoon  of  B.  communis,  which  has  a  very  pointed 
but  somewhat  larger  head  than  that  of  R.  fusca,  appears  never- 
theless to  be  able  at  times  to  penetrate  the  eggs  of  R.  fusca  and 
to  fertilize  them.  That  the  spermatozoon  of  Triton  can  enter 
the  eggs  of  R.  fusca  is  explained  very  easily  when  we  remember 
that  the  sharp  thin  head  of  the  Triton  spermatozoon  is  best 
adapted  of  all  species  to  penetrate  any  Qgg.  We  see,  too,  that 
the  thick-headed  spermatozoon  with  a  blunt  anterior  end,  such 
as  those  of  R.  arvalis  and  R.  esculenta,  cannot  fertilize  the  eggs 
of  any  other  species.  And  finally,  to  confirm  the  conclusion, 
we  find  that  these  two  species,  R.  arvalis  and  R.  esculenta, 
which  have  large-headed  spermatozoa,  are  alone  capable  of 
reciprocal  crossing.  Pfiiiger  believed  that  the  eggs  have  the 
greatest  capacity  for  cross-fertilization  at  the  height  of  the 
breeding  season,  and  the  same  statement  holds,  but  in  a  much 
less  degree,  for  the  spermatozoa. 

Experiments  on  Other  Forms 

Hertwig  has  objected  to  Pfliiger's  conclusions  on  the  ground 
that  the  eggs  of  the  sea-urchin  are  much  more  capable  of  cross- 
fertilization  after  they  have  begun  to  suffer  change  either  from 
being  kept  some  time  in  sea- water,  or  from  the  application  of 
drugs.  He  thought  that  the  frogs  kept  by  Pfiiiger  had  been 
also  under  artificial  conditions.  Further,  Hertwig  concluded, 
from  his  results  on  sea-urchins,  that  the  possibility  of  crossing 
does  not  depend  entirely  upon  the  external  conditions,  but  to 
a  large  extent  upon  some  unknown  property  of  the  Qgg.  Eggs 
in  good  condition  are  able  to  prevent  the  entrance  of  foreign 
spermatozoa,  but  as  soon  as  they  begin  to  lose  their  irritability, 
they  can  no  longer  resist  the  entrance. 


Ch.  Ill]       EXPERIMENTS  IX  CROSS-FERTILIZATION  29 

Born  obtained  some  interesting  results  as  to  the  relations 
existing  between  the  number  of  spermatozoa  in  a  fluid-extract 
of  the  testis  and  the  power  of  the  fluid-extract  to  fertilize  eggs. 
He  insists  that  in  some  cases  there  is  a  necessary  connection 
between  the  two.  It  is  far  from  clear  how  this  is  possible,  and 
the  result  may  depend  on  other  causes  which  are  introduced 
along  with  the  solutions  employed.  Moreover,  the  further 
question  of  polyspermy  of  such  eggs  complicates  the  results. 
Born  believes  that  many  cases  of  irregular  segmentation  of 
crossed  eggs  are  due  to  the  entrance  of  several  or  many  sper- 
matozoa into  the  egg^  which  act  as  centres  for  protoplasmic 
accumulations.  Such  a  segmentation  he  calls  "barock"  seg- 
mentation. On  the  other  hand,  Pfliiger  suggests  that  the 
irregular  cleavage  of  certain  of  the  crossed  eggs  is  the  result 
of  the  disintegration  of  the  male  pronuclei,  so  that  the  chro- 
matin is  scattered,  and  then  acts  on  the  protoplasm,  producing 
an  irregular  division. 

Recent  results  have  shown  that  polyspermy  is  a  normal  oc- 
currence in  some  amphibian  eggs,  and,  despite  the  presence 
of  several  spermatozoa,  normal  cleavage  and  normal  embryos 
result.  The  changes  that  take  place  within  the  cross-fertilized 
eggs  must  be  more  carefully  studied  before  a  final  decision  can 
be  reached  in  regard  to  the  meaning  of  some  of  the  experiments 
described  above. 

We  must  not  confuse  two  factors  that  enter  into  the  problem 
of  cross-fertilization.  On  the  one  hand,  the  spermatozoon  may 
not  be  able  to  push  through  the  gelatinous  coatings  of  the  egg, 
or  it  may  not  be  able  to  bore  through  the  outer  surface  of  the 
egg  itself,  or  it  might  be  unable  to  enter  the  protoplasm  if  the 
latter  were  entirely  free  from  its  coats. ^  On  the  other  hand, 
even  if  the  spermatozoon  could  successfully  enter  and  combine 
with  the  female  pronucleus,  it  does  not  follow  that  the  egg 
would  develop.  We  now  know  that  so  many  factors  enter 
into  the  problem  of  fertilization  of  the  egg  that  it  is  not  sur- 
prising when  we  find  that  two  pronuclei  that  have  ever  so 
slight  differences  are  not  able  to  carry  out  the  complicated 
machinery  of  cell-division  and  development. 

1  As  in  the  case  of  naked  pieces  of  protoplasm  of  the  egg  of  species  of  sea-urchins. 


30  DEVELOPMENT   OF   THE   FROG'S   EGG  [Ch.  Ill 

The  eggs  of  the  starfish  can  be  fertilized  by  the  spermatozoa 
of  the  sea-urchin,  —  forms  much  more  different  than  any  two 
species,  genera,  or  even  families  of  frogs,  and  the  early  stages 
of  segmentation,  and  the  formation  of  a  swimming  bias  tula  and 
gastrula  may  be  passed  through;  but  the  later  embryonic  devel- 
opment is  not  carried  out,  and  after  a  time  the  gastrulas  die.^ 

Hertwig's  experiments  ('77)  on  polyspermy  in  the  eggs  of 
echinoderms  show  that  when  several  spermatozoa  enter  the 
same  egg  a  karyokinetic  spindle  is  formed  around  each  of  the 
resulting  male  pronuclei  and  many  or  all  of  the  pronuclei 
divide.  Often  the  spindles  are  so  near  together  that  they 
mutually  influence  one  another  and  most  complicated  karyo- 
kinetic figures  result.  Subsequently  the  protoplasm  breaks  up 
around  the  pronuclei  in  a  most  irregular  way,  and  generally 
such  eggs  do  not  give  rise  to  even  the  earliest  stages  of  devel- 
opment. The  phenomenon  is  so  similar  to  the  "barock"  seg- 
mentation of  the  frog's  egg  that  it  seems  possible  that  in  the 
latter  the  result  is  brought  about  in  the  same  way  as  in  the 
echinoderms. 

Experiments  of  Rauber  and  of  Boveri 

Rauber,  in  1886,  tried  to  carry  out  the  following  interesting 
experiment.  The  segmentation-nucleus  of  a  frog's  egg^  one 
hour  after  fertilization,  was  removed  by  means  of  a  fine  pipette. 
The  same  process  was  carried  out  with  a  toad's  egg.  The 
nucleus  of  the  toad's  egg  was  then  placed  in  the  frog's  egg 
that  had  had  its  nucleus  removed,  and  the  nucleus  of  the 
frog's  egg  was  placed  in  the  toad's  egg.  Unfortunately, 
neither  egg  developed.  The  results  of  such  an  experiment 
would  be  of  the  greatest  importance  if  the  experiment  could 
be  successfully  carried  out ;  for  in  this  way  we  should  hope  to 
discover  whether  the  characters  of  the  embryo  come  from  the 
nucleus  or  from  the  protoplasm  of  the  egg. 

Boveri,  in  1889,  made  somewhat  similar  experiments  wdth 
the  egg  of  the  sea-urchin.  When  the  eggs  are  shaken  in  a 
small  tube,  they  are  broken  into  fragments,  some  with  nuclei 
and  others  without.     When  a  sufficiently  large  non-nucleated 

1  Morgan,  '93,  A7iat.  Anzeiger. 


Ch.  Ill]       EXPERIMENTS  IN   CROSS-FERTILIZATION  31 

fragment  is  penetrated  by  one  spermatozoon,  the  fragment 
develops.  Such  a  fragment  contains  only  half  the  number  of 
chromosomes  of  the  normal  fertilized  egg.^  Boveri  isolated 
some  of  these  fragments,  and  said  that  they  give  rise  to  small 
embryos  normal  in  structure.  Boveri  stated,  further,  that  if  a 
non-nucleated  fragment  of  the  egg  of  one  species  of  sea-urchin 
is  entered  by  one  spermatozoon  of  another  species,  the  result- 
ing larva  is  like  the  larva  of  the  father  (i.e.  it  is  like  the  larva 
of  the  individual  from  which  the  spermatozoon  comes).  If 
this  result  should  prove  true,^  it  would  show  that  the  nucleus 
and  not  the  protoplasm  determines  the  character  of  the  larva. 

1  Morgan,  '05,  Anat.  Anzeiger. 

2  Seeliger  ('95)  and  myself  ('95)  have  repeated  Boveri's  experiment  and  have 
tried  to  show  that  the  evidence  on  which  Boveri  based  his  conclusion  in  regard 
to  the  paternal  character  of  the  crossed  larva  is  insufficient. 


CHAPTER   IV 

CLEAVAGE   OF   THE  EGG 

When  the  egg  comes  to  rest  in  its  membranes  after  fertiliza- 
tion has  taken  place,  it  will  be  found  that  the  egg-axis  assumes 
an  oblique  position  with  respect  to  the  vertical.  The  degree  of 
obliquity  may  be  different  for  the  eggs  of  different  species  of 
frogs,  but  in  some  species  it  is  carried  so  far  that,  when  the 
egg  is  looked  at  from  above,  a  crescent  of  the  white  hemisphere 
can  be  seen  on  one  side  of  the  egg.  Roux  has  stated  that  the 
declination  of  the  egg-axis  takes  place  only  after  the  entrance 
of  the  spermatozoon,  and  toward  that  side  into  which  the  sper- 
matozoon has  penetrated.  1  He  was  able  to  determine  this  by 
artificially  fertilizing,  the  egg  at  definite  points.  By  means  of 
a  small  pipette,  water  containing  spermatozoa  was  brought  in 
contact  with  the  jelly  somewhere  near  the  upper  hemisphere  of 
an  egg.  Presumably  the  spermatozoon  will  then  take  the  short- 
est path  to  the  egg.  Roux  found  that  the  egg  after  a  time  gen- 
erally rotated  on  its  axis  toward  the  point  at  which  the  artificial 
fertilization  was  supposed  to  have  taken  place. 

Normal  Cleavage 

The  first  furrow  appears  on  the  egg  about  two  and  one  half 
to  three  hours  after  fertilization,  the  time  depending  in  part  on 
the  temperature  of  the  water.  A  rather  wide  furrow  appears 
in  the  flattened  area  near  the  black  pole,  and  rapidly  extends 
over  the  upper  surface  of  the  egg,  and  then  moves  more 
slowly  over  the  lower  or  white  surface.  The  sides  of  the 
furrow  are  often  wrinkled,  probably  a  mechanical  result  of  the 

1  Roux  believes  the  obliquity  to  be  a  usual  phenomenon  after  fertilization 
for  some  species ;  in  others  the  obliquity  is  only  occasionally  seen.  Schultze 
finds  it  to  be  as  much  as  forty-five  degrees  in  Rana  fusca. 

32 


Ch.  IV] 


CLEAVAGE   OF   THE   EGG 


33 


infolding  of  the  outer  harder  crust  of  the  egg.  These  wrinkles 
are  best  seen  in  the  upper  hemisphere ;  subsequently  they  dis- 
appear. It  will  be  found  on  cutting  in  two  an  egg  in  the 
process  of  cleavage  that  the  furrow  is  also  extending  throvgh 


B 


D 


H 


Fig.  12.  —  Segmentation  of  egg  and  formation  of  blastopore  (H,  \) .  A.  Eight-cell 
stage.  B.  Beginning  of  sixteen-cell  stage.  C.  Thirty-two-cell  stage.  D.  Forty- 
eight-cell  stage  (unusually  regular) .  E,  F.  Two  sides  of  same  egg  in  later  cleavage. 
G.  Still  later  cleavage.  H.  Dorsal  lip  of  blastopore.  I.  Circular  blastopore  (with 
lower  pole  toward  observer) . 


the  protoplasm  of  the  egg^  i.e.  dividing  the  contents  into  two 
parts.  When  the  superficial  furrow  has  encircled  the  egg^  the 
substance  also  has  been  divided. 


34 


DEVELOPMENT  OF   THE   FROG'S   EGG 


[Cii.  IV 


If  a  series  of  sections  be  made  through  the  egg  at  different 
stages  in  the  process  of  cleavage,  we  shoukl  see  that  prior  to 
the  division  of  each  blastomere  the  nucleus  had  divided  into 
two  parts.     This  takes  place  by  the  ordinary  process  of  indirect 


Fig.  13.  —  Segmentation  of  egg  (two,  eight,  sixteen,  and  thirty-two  cell  stages,  after 
M.  Schultze),  as  seen  from  above.  A.  Two-cell  stage;  beginning  of  second  fur- 
rows. B.  Eight-cell  stage,  with  cross-furrow.  C,  D,  F,  G.  Sixteen-cell  stages. 
E.  Eight-cell  stage  (regular  type).    H.  Thirty- two-cell  stage. 


or  karyokinetic  division.  Half  of  the  chromatin  passes  to  one 
pole  of  the  nuclear  spindle,  and  the  other  half  to  the  other  pole. 
As  the  spindle  elongates,  it  carries  with  it  the  surrounding  pig- 


Cii.  IV]  CLEAVAGE   OF   THE   EGG  35 

ment.  The  first  cleavage-plane  always  passes  directly  between 
the  separating  halves  of  the  segmentation-nucleus. 

There  is  an  infinite  number  of  possible  planes  through  which 
the  first  cleavage  might  divide  the  egg  into  equal  portions. 
What,  then,  determines  the  particular  plane  taken?  We  can 
think  of  this  plane  as  determined  by  external  conditions,  or 
by  the  internal  structure  of  the  egg,  or  by  a  combination 
of  the  two.  In  the  first  place,  it  seems  probable  that  at  the 
first  division  of  the  segmentation-nucleus  each  resulting  half 
will  get  half  of  the  chromatin  of  the  male  and  half  of  the  chro- 
matin of  the  female  pronucleus.  The  first  plane  of  division 
must  therefore  pass  at  right  angles  to  the  plane  of  apposition  of 
the  two  pronuclei.  That  is  to  say,  it  will  also  pass  through  the 
path  of  penetration  of  the  spermatozoon  (the  male  pronucleus), 
and  therefore  approximately  through  the  point  at  which  the 
spermatozoon  has  entered.  This,  according  to  Roux,  is  what 
actually  takes  place.  Moreover,  since  the  egg  has  rotated  as  a 
whole  in  the  direction  of  the  point  of  entrance  of  the  sperma- 
tozoon, the  first  cleavage  will  pass  exactly  through  the  highest 
point  of  the  white  crescent,  as  seen  from  above. 

On  the  other  hand,  there  is  no  direct  evidence  to  show 
that  the  two  apposed  pronuclei  retain  throughout  subsequent 
changes  the  position  of  first  apposition,  and  there  is  much  to 
show  that  in  the  frog's  egg,  as  well  as  in  other  eggs,  the  divid- 
ing nucleus,  or  the  direction  of  its  spnidle,  is  very  susceptible 
to  modifications  in  the  surrounding  conditions. 

Tliere  is  also  some  evidence  to  show  that  the  declination  of 
the  axis  of  the  frog's  egg  is  not  necessarily  determined  by  the 
entrance  of  the  spermatozoon,  but  by  the  arrangement  of  the 
internal  constituents  of  the  egg  itself.  If,  therefore,  it  could 
be  shown  that  the  declination  is  present  in  unfertilized  eggs, 
and  that  in  fertilized  eggs  the  plane  of  first  cleavage  passes 
more  or  less  through  the  highest  point  of  the  white  crescent, 
then  we  should  conclude  that  the  plane  of  first  cleavage  is  pre- 
arranged in  the  egg.  It  would  follow  as  a  corollary  that  the 
nuclear  spindle  orients  itself  with  respect  to  the  egg. 

There  is  direct  evidence  to  show  that  in  the  newt  some  such 
process  as  this  does  take  place.  Jordan  ('93)  has  shown  that 
the  spermatozoon  may  enter  at  any  point  of  the  surface  of  the 


36  DEVELOPMENT   OF   THE   FROG'S  EGG  [Ch.  IV 

upper  hemisphere,  yet  the  plane  of  first  division  is  always 
across  the  long  axis  of  the  egg.  Hence,  it  is  fair  to  assume 
that  the  segmentation-spindle  does  so  orient  itself  after  the 
fusion  of  the  male  and  female  pronuclei  that  half  of  the 
male  and  half  of  the  female  chromatin  are  carried  apart  in 
the  direction  of  the  long  axis  of  the  egg^  whatever  may  have 
been  at  first  the  position  of  apposition  of  the  two  pronuclei.  I 
have  dwelt  on  this  point  at  some  length  because  it  is  one  of 
great  importance  for  our  understanding  of  the  relation  betw^een 
egg  and  embryo  ;  and  because  it  is  much  to  be  desired  that  the 
present  state  of  doubt  should  be  cleared  away. 

After  the  protoplasm  has  divided  into  two  equal  parts,  the  egg 
"  rests  "  for  a  time.  During  the  division-period  the  hemispheres 
or  blastomeres  round  up  to  some  extent ;  but  as  soon  as  the 
division  is  completed  they  flatten  against  each  other,  so  that  the 
cleavage-plane  is  not  so  distinctly  seen  on  the  surface  of  the  egg. 
The  same  process  of  flattening  generally  takes  place  also  when 
the  dividing  egg  is  brought  into  preserving  fluids. 

During  the  time  of  division  we  may  speak  of  each  blastomere 
as  tending  to  become  itself  a  sphere,  but,  owing  to  the  lack  of 
room,  the  rounding  of  the  two  parts  is  very  imperfect.  In  other 
eggs  (e.^.  the  eggs  of  the  sea-urchin),  where  it  is  possible  to 
remove  the  egg-membranes,  it  has  been  found  that  then  each 
of  the  blastomeres  approaches  more  nearly  the  spherical  form, 
or  even  becomes  a  complete  sphere.  We  see  from  this  that  the 
external  conditions  may  at  least  modify  the  form  of  cleavage 
of  the  egg. 

It  is  sometimes  said  that  during  the  division  the  two  new 
parts  or  blastomeres  tend  to  repel  each  other  until  after  the 
division  is  completed,  and  to  attract  each  other  after  the  divi- 
sion is  finished.  Such  a  statement  is,  however,  of  little  value,  and 
may  convey  an  entirely  wrong  impression  of  the  changes  taking- 
place.  One  thing  seems  to  be  certain,  that  during  the  division 
of  the  egg  the  spheres  or  cells  have  an  influence  on  one  another. 
Whether  unseen  protoplasmic  connections  weld  them  together, 
or  whether  it  is  merely  a  question  of  contact  action,  has  not  yet 
been  fully  determined.^ 

1  See  Roux's  experiments  on  cytotaxis  ('90). 


Ch.  iy]  cleavage  of  the  egg  37 

The  nucleus  of  each  blastomere  during  the  resting-period 
undergoes  a  series  of  changes,  the  so-called  reconstructive  pro- 
cess taking  place.  The  chromatin-granules  or  chromosomes 
are  again  surrounded  by  a  nuclear  membrane,  and  the  granules 
fuse  into  a  thread  or  network.  At  the  next  division  of  the 
egg  the  nuclear  chromatin  is  again  set  free  in  the  protoplasm 
by  the  absorption  of  the  nuclear  membrane.  A  spindle  is 
formed  and  the  chromatin  in  each  cell  is  again  exactly  halved. 

The  second  cleavage-furrow  appears  about  three-quarters  of 
an  hour  after  the  appearance  of  the  first.  Each  of  the  two 
blastomeres  divides  in  a  plane  at  right  angles  to  the  preced- 
ing division.  The  furrows  begin,  generally  simultaneously,  in 
the  upper  hemisphere  of  each  of  the  first  two  blastomeres,  and 
push  toward  the  lower  pole  (Fig.  13,  A).  The  upper  and 
lower  ends  of  these  new  cleavage-planes  are  sometimes  exactly 
opposite  to  each  other,  so  that  the  effect  is  as  though  the  whole 
egg  had  been  divided  by  a  single  furrow  in  a  plane  at  right 
angles  to  the  first.  In  many  cases,  however,  the  new  planes  of 
division  are  not  quite  opposite,  but  reach  the  upper  and  lower 
poles  of  the  egg  at  different  points  along  the  first  plane 
of  division.  A  "  cross-line "  is  thus  formed.  The  same  re- 
sult may  be  brought  about  even  subsequent  to  division  by  a 
shifting  or  readjustment  of  the  blastomeres  on  one  another. 
As  a  rule,  when  a  cross-line  occurs  in  the  upper  pole,  another 
one  is  formed  in  the  lower  pole,  and  the  two  stand  in  space  at 
right  angles  to  each  other,  as  is  shown  in  tlie  diagrammatic 
reconstruction  in  Fig.  14,  A.  The  same  result  can  be  obtained 
by  compressing  four  clay  spheres  together  until  a  single  sphere 
results.  It  will  be  found  in  such  a  model  that  the  cross-lines 
above  and  below  are  generally  at  right  angles  to  each  other. 

The  third  furrows  come  in  at  right  angles  to  the  preceding 
planes  of  division,  and  are  therefore  horizontal  (Fig.  12,  A). 
The  third  planes  of  division  do  not  lie  at  the  equator  of  the 
egg^  but,  taken  together,  form  a  small  circle  in  the  black  hemi- 
sphere or  on  the  border-line  between  the  black  and  white  areas. 
Above  there  are  four  smaller  dark  blastomeres  and  below  four 
larger  white  blastomeres.  The  four  upper  blastomeres  are  of 
approximately  the  same  size,  but  in  some  species  of  frogs  it 
seems  that  one  is  a  little  smaller  than  the  rest,  one  is  somewhat 


38 


DEVELOPMENT  OF  THE  FROG'S  EGG 


[Ch.  IV 


larger,  and  two  are  intermediate  in  size.  The  smallest  hlasto- 
meres  of  the  upper  four  always  lie  nearest  the  summit  of  the  white 
crescent;  the  largest  is  its  vis-d-vis.  If  we  think  of  the  third 
planes  of  cleavage  as  lying  in  a  single  plane  not  quite  at  right 
angles  to  the  first  and  second,  but  tilted  a  little,  we  get  a  clearer 
conception  of  the  conditions  present.  The  fourth  cleavage- 
period  comes  in  from  a  half  to  three-quarters  of  an  hour  later. 
As  an  idealized  form,  we  may  think  of  the  new  planes  as  form- 
ing two  great  circles  at  right  angles  to  each  other,  and  lying 
vertically  and  between  the  planes  of  the  first  and  second  cleav- 


B 

Fig.  14.— a.  Diagram  of  four-cell  stage  to  show  cross-line, 
of  two  eggs.     (After  Rauber.) 


B,  C.  Sixteen-cell  stage 


ages  (Fig.  13,  C).     This  regular  form  is  rarely  if  ever  attained, 
and  the  greatest  amount  of  variation  is  found  to  exist. 

Remak  said  that  frog's  eggs  divide  much  more  regularly 
when  carried  in  from  places  where  they  were  normally  laid. 
If  allowed  to  stand  quietly  after  being  laid,  they  soon  begin  to 
divide  irregularly.  Vogt  has  also  observed  that  those  eggs  of 
the  salmon  develop  most  regularly  that  have  been  kept  in 
motion.  It  does  not  seem  probable,  however,  that  the  motion 
itself  could  have  an3'thing  to  do  directly  with  the  matter ;  but  if 
the  Qgg  be  not  supplied  with  a  sufficient  amount  of  fresh  water, 


Ch.  IV]  CLEAVAGE   OF   THE   EGG  39 

etc.,  it  might  no  doubt  segment  irregularly,  or  it  may  be  that 
the  motion  equalizes  the  external  conditions  so  that  the  eggs  keep 
a  more  nearly  spherical  shape  and  hence  divide  more  regularly. 

Max  Schultze  ('63)  and  Rauber  ('82)  have  made  the  most 
carefnl  study  of  the  variations  in  the  planes  of  division  of  the 
fourth  cleavage.  In  Figs.  13,  D,  F,  G,  and  14,  B,  C,  are  shown 
the  upper  hemispheres  of  several  eggs.  If  we  examine  the 
position  of  the  planes  of  the  last  (fourth)  divisions,  we  see  that 
in  the  upper  hemisphere  each  new  cleavage-plane  fails  generally 
to  reach  the  black  pole  of  the  egg^  but  passes  to  one  or  to  the 
other  side.  In  the  lower  hemisphere  the  new  planes  fall  far 
short  of  the  white  pole. 

Occasionally  we  find  eggs  in  the  upper  hemisphere  of  which 
one  or  more  of  the  fourth  planes  reach  the  black  pole  itself, 
and,  therefore,  lie  more  nearly  radial  in  position  (Fig.  13,  C). 
In  other  cases,  however,  one  or  more  of  the  new  fourth  cleav- 
age-lines may  even  be  nearly  in  the  horizontal  plane.  In  the 
lower  pole  also  there  is  much  variation,  and  occasionally  a  blas- 
tomere  is  divided  into  a  very  small  and  a  very  large  part,  owing 
to  the  sudden  turning  aside  of  the  new  cleavage-line,  so  that 
it  meets  one  of  the  first  two  cleavage-planes  before  it  has 
extended  far  into  the  lower  hemisphere.  Other  eggs  at  this 
same  stage  show  a  strictly  bilateral  arrangement  of  the  cells 
in  the  upper  hemisphere.  In  Fig.  13,  D,  F,  G,  we  see  that 
the  fourth  cleavage-planes  have  met  the  same  furrow  (first  or 
second).  Also  in  Fig.  14,  B,  a  bilateral  symmetry  is  present, 
formed  in  a  somewhat  different  way,  as  the  figure  shows  ;  and 
in  this  egg  the  lower  hemisphere  also  is  symmetrically  divided. 

During  the  fifth  cleavage-period  the  irregularities  in  the 
division  of  the  cells  is  generally  so  great  that  we  cannot  speak 
definitely  of  any  special  direction  of  the  new  planes.  Never- 
theless there  is  a  tendency  for  some  of  the  new  furrows  to 
come  in  at  right  angles  to  the  last  planes  of  division.  There- 
fore, many  and  occasionally  all  of  the  new  fifth  cleavage-planes 
are  horizontal  (Fig.  12,  C).  The  eight  cells  in  the  upper  hemi- 
sphere divide  into  equal  or  nearly  equal  parts,  but  the  eight 
blastomeres  of  the  lower  hemisphere  divide  unequally  into  eight 
upper  smaller  blastomeres  containing  pigment,  and  eight  lower 
blastomeres  which  are  the  white  blastomeres  around  the  loAver 


40 


DEVELOPMENT  OF  THE  FROG'S  EGG     [Ch.  IV 


pole.  The  division  of  the  lower  eight  blastomeres  is  sometimes 
so  regular  that  a  circle  of  eight  dark  cells  is  formed  around  the 
equator  of  the  egg  (Fig.  12,  C).  After  this  division  thirty- 
two  cells  are  present.  At  this  period  the  distribution  of  the 
cells  over  the  dark  and  light  regions  of  the  egg  is  such  that 
the  cells  on  the  side  of  the  egg  showing  the  light  crescent  above 
are  smaller  than  the  corresponding  cells  on  the  opposite  side. 


Fig.  15.  —  Stages  in  the  segmentation  (A,  B,  C)  of  the  egg,  and  the  beginning  of 
gastrulation  (D).    AR.  Beginning  of  archenteron.     SG.  Segmentation-cavity. 


Up  to  the  present  time  all  the  divisions  of  the  cells  started 
on  the  outside  of  the  egg  or  blastomeres  and  progressed  inward. 
As  early  as  the  eight-cell  stage,  a  cavity  appeared  between  the 
cells  in  the  upper  hemisphere  of  the  egg.  This  is  the  "seg- 
mentation-cavity."    It  is  filled  with  an  albuminous  fluid  which 


Ch.  IV]  CLEAVAGE   OF   THE   EGG  41 

probably  comes  entirely,  or  in  part,  from  the  surrounding  cells. 
This  cavity  gets  larger  and  larger  as  development  proceeds 
(Fig.  15,  A).  If  an  egg  be  cut  open  after  the  thirty-two-cell 
stage,  it  will  be  found  that  many,  perhaps  all,  of  the  cells  or 
blastomeres  are  undergoing  division  into  outer  and  inner  cells 
(Fig.  15,  B).  We  may  speak  of  this  process  as  a  delamination. 
Before  the  egg  has  divided  into  sixty-four  cells,  as  seen  on  the 
surface,  this  delamination  into  inner  and  outer  cells  has  in  most 
cells  taken  place. 

The  cell-divisions  now  proceed  more  rapidly  and  with  great 
irregularity.  The  rhythm  also  soon  becomes  lost,  so  that 
wliile  some  cells  are  dividing  others  are  resting.  Not  only 
have  the  outer  blastomeres  continued  to  divide  at  the  surface, 
but  also  below  the  surface  of  the  egg  new  blastomeres  are  being 
cut  off  from  the  outer  cells;  the  inner  blastomeres  also  continue 
to  divide.  In  the  upper  part  of  the  Qgg  a  large  segmentation- 
cavity  forms.  Its  roof  is  covered  by  several  layers  of  small 
deeply  pigmented  cells,  its  sides  by  larger  cells,  and  its  floor  is 
formed  by  the  large  whitish  yolk-bearing  cells  (Fig.  15,  C). 

If  the  surface  of  the  egg  be  carefully  examined  during  these 
later  stages,  it  will  be  found  that  the  cells  over  one  side  are  dis- 
tinctly smaller  than  those  over  the  opposite  side.  We  see  that 
the  side  of  the  egg  containing  the  most  pigment  is  made  up  of 
larger  cells.  In  Fig.  12,  G,  H,  the  opposite  sides  of  an  egg  are 
shown,  and  here  the  less  pigmented  cells  are  seen  to  be  smaller 
than  the  cells  in  the  same  position  on  the  other  side  of  the  egg. 
Sections  show,  moreover,  that  this  difference  in  size  is  not  only 
found  on  the  surface  of  the  egg^  but  also  in  the  interior  as  well. 
During  the  early  periods  of  cleavage  the  egg  has  become 
neither  more  nor  less  pigmented  on  its  surface,  and  has  retained 
the  same  distribution  of  pigment  as  in  the  unsegmented  egg. 

Besides  the  variations  in  the  cleavage  noted  above,  others  are 
more  rarely  found  that  depart  much  further  from  the  usual 
typical  forms.  The  first  furrow,  for  instance,  may  divide  the 
egg  into  very  unequal  parts.  The  second  furrow  may  appear 
before  the  first  has  reached  the  lower  pole.  The  third  furrows 
may  stand  vertically^  passing  from  near  the  upper  pole  into  the 
lower  hemisphere,  i.e.  the  third  furrows  occupy  the  position  of 
the  fourth  furrows  of  the  usual  type  of  cleavage. 


42  DEVELOPMENT  OF  THE   FROG'S  EGG  [Ch.  IV 

Correspondence  of  the  First  Cleavage-plane  and  the 
Median  Plane  of  the  Embryo 

If  the  egg  when  in  the  two-cell  stage  be  fixed  ^  so  that  it  can- 
not rotate  in  a  horizontal  plane,  and  if  such  an  egg  be  carefully 
watched  until  the  moment  when  the  medullary  folds  have  just 
appeared,  it  will  be  found  that  the  position  of  the  plane  of  first 
cleavage  corresponds  approximately,  or  even  exactly,  to  the 
median  plane  of  the  body  of  the  embryo.  This  experiment 
was  first  made  by  Newport  in  1851,  subsequently  by  Pfliiger 
('83),  and  Roux  ('85),  and  later  by  other  workers. ^  If,  how- 
ever, during  the  subsequent  cleavage-periods,  i.e.  during  the 
eight  and  sixteen  cell  stages,  etc.,  the  position  of  this  plane  be 
kept  in  mind,  it  will  be  found  that  the  later  blastomeres  from 
one  or  the  other  side  often  pass  over  the  imaginary  plane  that 
corresponds  to  the  plane  of  first  division.  Striking,  therefore, 
as  is  the  coincidence  of  the  first  plane  of  cleavage  and  the 
middle  plane  of  the  embryo,  it  remains  to  be  proved,  I  think, 
that  there  is  any  direct  causal  connection  between  the  first 
cleavage-plane  and  the  median  line  of  the  body.  It  may  be 
that  the  two  phenomena  are  coincident  because  the  internal 
arrangement  of  the  egg  that  determines  one  may  also,  but 
independently,  determine  the  other.  In  the  newt  Jordan  ('93) 
has  shown  that  the  first  plane  of  cleavage  corresponds  approxi- 
mately to  the  cross-plane  of  the  body.  That  is,  the  first  two 
blastomeres  correspond  to  the  anterior  and  posterior  parts  of  the 
body  respectively.  He  suggests  that  the  shape  of  the  egg-cap- 
sule of  the  newt  may  be  the  cause  determining  the  plane  of  first 
division.  Some  other  factor  than  that  of  the  position  of  the 
first  plane  of  cleavage  seems  to  determine  the  position  of  the 
embryo  on  the  egg,  for  in  the  teleost's  egg,  where  the  sym- 
metry and  bilaterality  of  the  cleavage  is  even  more  sharply 
marked  than  in  the  frog  or  newt,  there  seems  to  be  no  relation 
at  all  between  the  first  cleavage-planes  and  the  planes  of  the 
adult  body. 3 

1  For  method,  see  Pfluger  ('83),  Roux  ('85),  and  Morgan  ('91). 

2  Rauber  ('86)  has  later  contradicted  these  results,  but  it  is  probable  that 
there  is  an  error  in  his  experiment. 

3  Clapp  ('91),  Morgan  ('93). 


Ch.  IV] 


CLEAVAGE   OF   THE   EGG 


43 


Roux's  Experiments  with  Oil-drops 

The  arrangement  assumed  by  the  blastomeres  after  each 
cleavage  has  attracted  much  attention.  A  system  of  soap- 
bubbles,  or  of  balls  of  clay  compressed  into  a  sphere,  gives 
somewhat  similar  figures.  In  this  connection  Roux  has  made 
a  most  instructive  series  of  experiments.  A  small  wine-glass  is 
half  filled  with  dilute  alcohol  and  then  sufficient  oil  is  poured 
in  to  form  a  large  drop.  A  stronger  (lighter)  alcohol  is  now 
poured  on  top  of  the  oil,  which  assumes  a  spherical  or  nearly 
spherical  shape.      The  drop  lies  suspended  between  the  two 


X 

a' 

b'       Xv 

/  . 

\ 

r    * 

N 

>/|>        ^ 

\' 

/] 

\     ^      1 

\ 

A 

b\^ 

Fig.  16.  —  Systems  of  oil-drops, 
marked  A',  B' 


(After  Roux.)     In  C,  the  lowest  drops  should  be 
;  and  those  next  them  A",  B". 


alcohols  and  its  periphery  just  touches  the  walls  of  the  glass. ^ 
It  is  possible  to  divide  this  sphere  of  oil  into  equal  or  unequal 
parts  by  means  of  a  glass  rod  and,  if  precautions  are  taken,  the 
drops  will  not  for  a  time  flow  together.     The  drops  tend  each 


1  Roux  recommends  olive  or  paraffine  oil.  I  find  that  thick  cotton-seed  oil 
gives  as  good  or  better  results  when  suspended  between  fifty  and  seventy  per 
cent,  alcohols.  A  smaller  drop  is  to  be  used  when  more  than  two  divisions  are 
to  be  made. 


44        DEVELOPMENT  OF  THE  FROG'S  EGG     [Ch.  IV 

to  become  spherical,  but  the  wine-glass  holds  them  together 
and  their  surface-tensions  cause  them  to  assume  definite  rela- 
tions to  one  another.  When  the  drop  is  divided  into  two 
equal  parts,  the  halves,  if  little  compressed,  arrange  themselves 
as  shown  in  Fig.  16,  A,  and  if  much  compressed,  as  shown  in 
Fig.  16,  E. 

If  each  drop  is  again  divided,^  the  resulting  four  drops 
arrange  themselves  as  shown  in  Fig.  16,  B.  A  large  central 
cavity,  similar  to  the  segmentation-cavity  of  many  segmenting 
eggs,  is  present  in  the  centre  between  the  four  drops. 

If  the  drops  had  been  first  divided  unequally,  we  should  find 
that  the  smaller  drop,  having  a  stronger  tendency  to  become 
round,  caused  the  region  of  contact  of  the  two  drops  to  bend 
in  toward  the  larger  drop.  If  each  of  these  two  drops  is 
again  divided,  the  four  parts  arrange  themselves  as  shown  in 
Fig.  16,  D.  The  same  result  is  brought  about  if  we  divide 
the  drop  at  first  equally  and  then  each  of  the  products  un- 
equally (Fig.  16,  D). 

If  we  first  divide  a  drop  equally  and  then  each  of  the  two 
unequally,  but  at  different  ends  of  each  drop,  as  shown  by  the 
dotted  lines  in  Fig.  16,  E,  the  resulting  four  drops  arrange 
themselves  as  shown  in  Fig.  16,  F.  Moreover,  and  this  is  a 
point  of  much  importance,  it  is  a  matter  of  indifference  in 
what  direction  the  smaller  drops  are  cut  off.  For  instance, 
if  each  of  the  first  two  drops  is  divided  along  the  dotted  lines, 
a-a,  a-a  (Fig.  16,  E),  the  result  is  the  same  as  when  the  divi- 
sion takes  place  along  the  line  h-h^  h-h.  In  either  case  the 
drops  arrange  themselves  as  shown  in  Fig.  16,  F.  The  two 
larger  drops  come  together  at  the  centre  of  the  system  and 
flatten  somewhat  against  each  other,  producing  a  cross-line. 
The  two  smaller  drops  are  pushed  out  more  toward  the 
periphery  of  the  system. 

If  we  adopt  the  method  of  lettering  shown  in  Fig.  16,  B, 
we  can  follow  more  readily  the  further  divisions.  Divid- 
ing equally  two  of  the  four  drops  of  Fig.  16,  B,  we  find  the 


1  In  dividing  the  drops  it  is  better  to  move  the  rod  always  from  the  centre 
toward  the  periphery.  The  plane  of  the  first  division  is  indicated  in  the  fig- 
ures by  the  heavier  line. 


Ch.  IV] 


CLEAVAGE   OF   THE   EGG 


45 


arrangement  of  the  six  resulting  drops  to  be  that  shown  in 
Fig.  IT,  A.  We  can  write  out  the  arrangement  in  the  form 
of  equations:  thus  (in  Fig.  16,  B)  a  =  A  =  B  =  h;  and  (in 
Fig.  IT,  A)  h'  =  h",  a'  =  a". 

Dividing  unequally  a  and  h  so  that  a'  is  less  than  a"  and  h'  is 
less  than  b'\  the  drops  arrange  themselves  as  shown  in  Fig. 
IT,  B.  A  large  central  cavity  is  present  in  the  centre  of  the 
system. 

If  each  of  the  four  equal  drops  (Fig.  16,  B)  be  equally 
divided,  the  resulting  eight  drops  arrange  themselves  as  shown 


a' 

«•'    . 

K 

/    *" 

L 

J 

■•) 

\  ^" 

U 

B 

K^ 

B 


D  E  F 

Fig.  17.— Sj'stems  of  oil-drops.     (After  Roux.) 


in  Fig.  16,  C.  A  central  cavity  is  present,  but  smaller  than 
when  only  four  equal  drops  formed  the  system. 

If  we  divide  each  of  four  equal  drops  (Fig.  16,  B)  ujiequally 
so  that  a"  is  less  than  a'  and  h"  is  less  than  h\  also  A"  is  less 
than  A'  and  B"  is  less  than  B\  the  resulting  eight  drops 
arrange  themselves  as  shown  in  Fig.  IT,  C.  The  four  larger 
drops  come  together  in  the  centre,  pushing  the  smaller  drops 
more  toward  the  periphery  of  the  sj^stem. 

If  we  divide  four  equal  drops  (Fig.  16,  B)  so  that  a"  is  less 


46        DEVELOPMENT  OF  THE  FROG'S  EGG     [Ch.  IV 

than  a\  h"  is  less  than  h\  but  A'  is  less  than  A^'  and  B'  is 
less  than  B'\  then  the  drops  assume  the  form  shown  in 
Fig.  IT,  D. 

If  we  divide  four  equal  drops  (Fig.  16,  B)  so  that  a'  is  less 
than  a",  h"  is  less  than  6',  A"  is  less  than  A' ^  and  B'  is  less  than 
J5",  the  resulting  eight  drops  arrange  themselves  as  shown  in 
Fig.  17,  E.  In  this  system  it  is  instructive  to  note  how  far 
the  first  division-plane  is  drawn  out  of  its  straight  course  as 
a  result  of  the  shifting  of  the  drops  on  one  another.  It  is  not 
unusual  for  one  of  the  drops  to  glide  into  the  centre  of  the 
system,  as  shown  in  Fig.  17,  F.  This  produces  a  more  stable 
arrangement  than  when  a  large  central  cavity  is  present. 

Most  of  these  systems  are  found  also  in  the  segmenting  frog's 
Qgg^  as  can  be  seen  by  comparing  these  figures  of  the  oil-drops 
with  the  figures  of  the  segmenting  frog's  Qg§,  (t'ig- 13)  by  Max 
Schultze  made  in  1863.  Rauber  has  also  given  figures  showing 
arrangements  of  the  upper  eight  blastomeres  (Fig.  11),  like 
the  systems  of  oil-drops  shown  in  Fig.  17,  D  and  E. 

A  careful  comparison  between  the  systems  of  oil-drops  and 
the  arrangement  of  the  blastomeres  of  the  frog's  Qgg  sliows, 
as  Roux  points  out,  that  while  in  many  cases  the  agreement 
is  perfect,  yet  occasionally  the  blastomeres  assume  an  arrange- 
ment that  oil-drops  of  the  same  size  would  not  assume.  For 
instance,  Roux  figures  an  arrangement  of  the  blastomeres  like 
that  of  Fig.  17,  C,  but  here  the  blastomere  corresponding  to  a' 
is  less  than  a"  and  A'  is  less  than  A" ,  In  this  Qg^  the  smaller 
blastomeres  meet  in  the  centre,  but  this  never  occurs  in  the 
system  of  oil-drops. 

Roux  removed  a  part  of  a  blastomere  so  that  it  became  sud- 
denly smaller.  A  new  arrangement  ought  now  to  have  taken 
place  among  the  blastomeres  if  they  conformed  entirely  to  the 
laws  regulating  the  oil-drops.  In  one  case  where  four  blasto- 
meres were  present,  the  blastomere  that  had  been  reduced  in 
size  did  move  out  more  toward  the  periphery  of  the  system, 
and  the  two  neighboring  blastomeres  pushed  in  more  toward 
the  centre  to  form  a  cross-line.  In  other  experiments,  how- 
ever, the  blastomeres  did  not  rearrange  themselves  in  con- 
formity with  the  systems  of  oil-drops.  For  instance,  in  one 
experiment  in  which  material  was  drawn  out  of  one  of  the  first 


Cii.  IV]  CLEAVAGE   OF   THE   EGG  47 

four  blastomeres,  the  inner  end  of  the  reduced  blastomere 
retained  its  central  position.  In  another  instance  material  was 
taken  out  of  that  one  of  the  four  blastomeres  that  had  already 
made  a  broad  cross-line  with  its  vis-d-vis.  Although  this 
blastomere  was  much  reduced  in  size  and  made  smaller  than 
any  other  blastomere  of  the  system,  yet  it  retained  the  same 
cross-line  as  before ;  i.e.  it  was  not  pushed  out  to  the  periph- 
ery. Even  when  the  experiment  w^as  made  at  the  time  of 
appearance  of  the  second  cleavage,  the  newly  forming  blasto- 
meres did  not  in  all  cases  adjust  themselves  in  agreement  with 
the  laws  regulating  the  oil-drops. 

These  results  show  that  the  conditions  present  in  the  frog's 
Qgg  do  not  allow  the  blastomeres  to  assume  always  the  arrange- 
ment shown  by  the  same  number  of  oil-drops  having  the  same 
relative  size.  Roux  points  out  several  differences  in  the  two 
cases.  The  walls  of  neighboring  blastomeres  seem  to  stick 
together,  and  this  would  prevent  the  blastomeres  from  gliding 
freely  over  one  another  should  any  change  take  place  to  disturb 
the  equilibrium.  Moreover,  the  blastomeres  are  living  con- 
tractile bodies,  and  through  their  ovm.  internal  activity  may 
interfere  with  the  mechanical  tendencies  of  the  system.  The 
nature  of  the  surface  of  each  blastomere  and  the  sort  of  changes 
taking  place  in  the  surface  may  also  affect  the  arrangement. 

It  will  be  seen  then,  as  has  been  said,  that  there  may  be  fac- 
tors present  in  the  frog's  Qgg  that  so  influence  the  arrangement 
of  the  blastomeres  that  the  systems  do  not  always  conform  to 
those  of  the  oil-drops.  Nevertheless,  the  results  from  the  latter 
give  us  an  ideal  scheme  showing  the  effect  produced  by  one  set 
of  factors,  —  that  of  surface-tension.  It  seems  highly  probable 
that  surface-tension  is  also  an  important  factor  in  the  segment- 
ing Qgg^  but  other  conditions  present  prevent  its  free  play. 

Historical  Account  of  the  Cleavage  of  the  Frog's 

Egg 

♦ 

The  earliest  observations  on  the  segmentation  of  the  animal- 
ovum  were  made  upon  the  frog's  Qgg.  Swammerdam  ('37) 
saw,  but  did  not  understand,  the  first  cleavage-furrow  of  the 
^gg'     Spallanzani,  in  1785,  observed  the  first  two  furrows  cross- 


48  DEVELOPxMENT   OF   THE   FROG'S  EGG  [Cii.  IV 

ing  each  other  at  right  angles.  Prevost  and  Dumas,  in  1824, 
gave  for  the  first  time  a  definite  description  of  the  cleavage  of 
the  frog's  egg.  They  described  the  first  furrow  beginning  in 
the  black  hemisphere  and  stretching  out  into  the  white  hemi- 
sphere. They  saw,  moreover,  the  small  lateral  creases  or  folds 
along  the  edges  of  the  first  cleavage-furrow.  The  second  fur- 
row, they  said,  cuts  the  first  at  right  angles.  When  the  dark 
hemisphere  is  divided  into  four  segments,  they  saw  that  then 
a  third  equatorial  furrow  forms  near  the  boundary  of  the  two 
hemispheres.  The  next  furrows,  they  said,  appear  parallel  to 
the  first. 

Rusconi  ('26)  observed  that  the  furrows  were  not  simply  sur- 
face-lines, but  cut  up  the  yolk  into  separate  parts,  producing 
finally  a  large  number  of  small  pieces,  which  he  believed  were  the 
elements  from  which  the  different  parts  of  the  body  developed. 
Von  Baer's  description  of  the  process,  in  1834,  is  much  more 
exact  than  the  accounts  of  his  predecessors.  His  interpretation, 
too,  is  much  clearer  and  nearer  to  the  truth.  He  said  that  the 
advance  of  the  first  furrow  into  the  lower  hemisphere  goes  on 
as  though  it  were  overcoming  great  difficulty.  The  tearing 
apart  of  the  yolk  into  halves  is  brought  about  as  a  result  of  a 
living  activity,  and  the  power  to  divide  the  ovum  does  not  reside 
only  in  the  surface  of  the  ovum,  but  extends  throughout  the 
whole  mass.  Von  Baer  noticed  that  after  the  division  the 
cross-diameter  of  the  egg  is  greater  than  the  vertical  diameter 
in  the  proportion  of  six  to  five,  and  he  said  that  the  difference 
would  be  greater  were  it  not  for  the  egg-membrane.  The  ten- 
dency, he  said  further,  is  to  form  two  spheres  which  are,  how- 
ever, compressed  against  each  other  by  the  membrane.  Since 
the  division  of  the  white  hemisphere  progresses  more  slowly, 
and  since  the  third  division  is  nearer  to  the  upper  hemisphere, 
we  can  understand  why  the  dark  portions  are  ahvays  smaller 
than  the  white  portions.  When  the  surface  appears  again 
smooth  (owing  to  the  smallness  of  the  portions  into  which  the 
ovum  has  divided),  the  egg  is  very  distinctly  larger  than  at 
first.  Von  Baer  concluded  that  material  is  taken  up  from  the 
outside  to  form  the  albumen,  and  hence  to  enlarge  the  ovum. 
He  interpreted  the  process  of  cleavage  as  the  self-division  of  the 
individual  to  form    innumerable   smaller   units.     In  the  later 


Cii.  IVJ  CLEAVAGE   OF   THE   EGG  49 

stages  these  smaller  bodies  fuse  by  a  vital  process  into  a  new 
whole,  and  a  new  individual  is  thus  produced  from  the  frag- 
ments of  the  first. 

Schwann  and  Schleiden  promulgated  the  cell-theory  in  1838- 
1839.  This  produced  an  effect  on  all  subsequent  interpreta- 
tions of  the  segmentation  of  the  frog's  egg.  The  main  points 
to  settle  were :  first,  whether  the  process  of  cleavage  is  a  pro- 
cess of  cell-division,  i.e.  whether  the  egg  is  a  cell  that  divides ; 
second,  whether  the  bodies  that  result  from  the  segmentation 
of  the  egg  pass  over  into  the  cells  of  the  embryo.  The  search 
for  the  nucleus,  before  and  after  the  process,  also  occupied  the 
attention  of  workers  on  the  subject.  Bergmann  ('41)  was  the 
first  to  treat  the  process  of  cleavage  from  the  cell  standpoint. 
The  first  divisions  of  the  egg  did  not  produce  true  cells,  he  said; 
yet  as  the  results  of  these  divisions  went  over  directly  into  the 
cells  of  the  embryo,  therefore  the  division  of  the  batrachian 
egg  is  the  introduction  of  cell-formation  into  the  yolk.  Later, 
he  said  that  the  yolk  may  be  thought  of  as  strongly  disposed 
to  form  cells,  but  that  nuclei  are  wanting.  Reichert's  in- 
terpretation ('46)  was  a  step  backwards.  Kolliker,  in  1843, 
described  the  segmentation-spheres  as  without  a  membrane 
and  containing  spore-like  bodies  which  multiplied  endoge- 
nously.  When  these  bodies  are  set  free,  he  thought,  they 
become  the  cells  from  which  the  tadpole  is  built  up.  Cramer 
('48)  thought  that  the  early  cleavage-spheres  formed  mem- 
branes (cell-walls)  and  were  the  progenitors  of  the  true  cells 
of  the  body.  Remak  ('o0-'55)  argued  that  the  cleavage-pro- 
cess was  the  beginning  of  cell-division,  and  that  the  products 
resulting  from  division  formed  the  cells  of  the  embryo.  This 
statement  marked  a  distinct  advance  and  is  the  standpoint 
taken  at  the  present  time.  ]\Ioreover,  Remak  thought  it  highly 
probable  that  there  was  a  continuity  of  the  original  egg-nucleus 
with  the  cleavage-nuclei.  Max  Schultze,  in  1863,  described 
admirably  the  process  of  cleavage  of  the  frog's  egg.  He  spoke 
of  the  egg  as  a  cell  with  protoplasm  and  nucleus,  and  of  the 
process  of  cleavage  as  cell-division.  Ordinary  cell-division 
depends,  he  said,  on  the  contractility  of  the  protoplasm.  The 
same  property  belongs  to  the  egg-yolk,  since  it  divides  like  a 
true  cell. 


CHAPTER   V 

EARLY  DEVELOPMENT   OF   THE  EMBRYO 

In  the  preceding  chapter  the  cleavage  of  the  egg  has  been 
described  to  the  period  when  the  bhistopore  is  about  to  appear 
on  the  surface.  During  the  subsequent  development  the  cells 
continue  to  divide,  so  that  at  no  time  can  the  cleavage  or  the 
cell-division  be  said  to  cease.  At  each  successive  stage  the 
number  of  cells  is  greater  than  in  the  preceding  stage.  This 
statement  does  not  imply,  however,  that  the  formation  of  each 
new  structure  is  introduced  by  new  cell-divisions  in  the  region 
where  the  change  is  about  to  begin,  because  many  changes  take 
place  in  regions  where  cell-division  is  not  more  rapid  than  else- 
where. 

The  spherical  form  of  the  "  egg  "  or  young  embryo  is  soon 
lost.  In  the  present  chapter  we  shall  follow  the  changes  that 
can  be  seen  taking  place  on  the  exterior  of  the  living  embryo ; 
and  in  the  following  chapter  we  shall  attempt  to  make  out  the 
movements  of  cells  and  groups  of  cells  that  take  place  in  the 
interior  of  the  embryo  during  this  period. 

The  Blastopore 

On  that  side  of  the  egg  where  the  smaller  cells  are  found,  a 
short  horizontal  line  of  pigment^  appears  amongst  the  white 
cells  below  the  equator  of  the  egg  (Fig.  12,  I).  This  line  marks 
the  beginning  of  the  archenteron,  and  the  cells  bounding  the 
upper  or  darker  side  of  the  pigment-line  form  the  dorsal  lip  of 
the  blastopore.  The  dorsal  lip  becomes  crescentic  in  outline, 
with  the  concavity  of  the  crescent  turned  toward  the  white 
hemisphere  (Fig.  19,  I,  II).     If  the  living  egg  be  watched,  it 


1  There  is  a  great  deal  of  variation  at  first  in  the  shape  of  the  blastopore. 

.    50 


Ch.  V]         EARLY   DEVELOPMENT   OF   THE   EMBRYO  51 

will  be  found  that  changes  take  place  at  this  time  in  the  blasto- 
poric  region  with  great  rapidity. 

Pfliiger  has  described  ('83)  these  changes,  and  we  may 
follow  his  admirable  account,  subsequently  adding  other  facts 
that  have  since  been  discovered.  The  eggs  wdiich  Pfliiger 
studied  1  were  taken  from  the  uterus  at  twelve  o'clock  midday, 
and  placed  in  a  row  on  a  thick  glass  mirror.  The  mirror  was 
then  put  into  a  dish,  and  water  added  to  the  depth  of  2  mm. 
In  this  way,  owing  to  the  reflection  of  the  lower  pole  by  the 
mirror,  both  hemispheres  of  the  egg  could  be  watched.  Dur- 
ing the  night,  when  the  temperature  was  low,  the  eggs  de- 
veloped more  slowly.  At  six  o'clock  in  the  morning  the 
thermometer  stood  at  16°  C.  At  this  time  the  eggs  showed 
on  the  lower  hemisphere,  and  in  the  upper  fourth  of  that  region 
and  therefore  just  beneath  the  equator,  the  first  trace  of  the 
dorsal  lip  of  the  blastopore.  By  ten  A.M.  the  long  horizontal 
split  (dorsal  lip  of  the  blastopore)  had  become  distinctly  marked 
as  an  indentation  of  the  surface  of  the  egg.  At  eleven  a.m., 
the  dorsal  lip  had  moved  somewhat  further  below  the  equator 
of  the  egg,  i.e.  toward  the  lower  pole.  The  ''split"  is  now 
broader,  and  its  corners  turned  down  so  that  it  forms  a  cres- 
cent, with  the  lower  pole  of  the  egg-axis  as  its  middle  point. 
The  diameter  of  the  crescent  is  to  the  egg-diameter  as  2:3. 
From  the  corners  of  the  crescent  a  furrow  continues  to  extend 
on  each  side  around  the  white  hemisphere.  The  progress  of 
the  dorsal  lip  toward  the  lower  pole  is  not  due  to  a  rotation 
of  the  egg  as  a  whole,  but  to  the  migration  of  the  dorsal  lip  over 
the  ivhite  hemisphere.  At  half-past  twelve  o'clock  (twenty- 
four  hours  after  fertilization),  the  dorsal  lip  has  progressed 
further  toward  the  lower  pole.  The  crescent  has  at  the  same 
time  extended  so  as  to  form  a  half-circle  whose  diameter  is 
somewhat  less  than  in  the  preceding  stage.  It  stands  now  in 
relation  to  the  diameter  of  the  egg  as  1  :  2. 

By  one  o'clock  p.m.,  the  semicircle  forming  the  dorsal  and 
lateral  lips  of  the  blastopore  has  extended  so  as  to  form  a 
complete  circle  (Fig.  19,  A,  IV).  The  white  yolk-cells  pro- 
trude  from  the  centre   of   this  circle  and   form  the  so-called 

^  Bufo  cinereus. 


52 


DEVELOPMEXT   OF   THE   FROG'S  EGG  [Ch.  V 


Fig.  18. — Diagrams  to  show  extent  of  movement  of  dorsal  lip  of  blastopore  and 
rotation  (F)  of  embryo.     (After  Pliiiger.) 


yolk -plug.     The  diameter  of  the  circle  around  the  yolk-plug  is 
still  smaller  than  before.     At  2.15  p.m.,  the  opening  containing 


Ch.  Y]         EARLY   DEYELOPMEXT   OF   THE   EMBRYO  53 

the  yolk-plug  —  the  so-called  opening  of  Rusconi,  or  blastopore 
—  is  still  smaller.  The  periphery  of  this  circular  blastopore  is 
deeply  pigmented.  At  4.15,  the  opening  is  further  reduced 
and  measures  no  more  than  one-eighth  of  the  diameter  of  the 
egg.  The  blastopore  will  now  be  found  to  have  progressed  so 
far  that  it  again  lies  just  beneath  the  equator  of  the  egg,  but  on 
the  side  of  the  egg  opposite  to  that  at  which  the  dorsal  lip  first 
appeared.  We  can  summarize  by  saying  that  the  dorsal  lip  of 
the  blastopore  has  moved  over  a  meridian  of  the  egg  from  a 
point  near  the  equator  across  nearly  to  the  opposite  point  of  the 
equator.  The  movement  takes  place  over  the  lower  white  hemi- 
sphere, and  during  the  process  the  position  of  the  egg  remains 
unchanged.  The  arc  traversed  by  the  dorsal  lip  of  the  blasto- 
pore is,  however,  not  as  much  as  180  degrees,  because  it  started 
below  the  equator  and  does  not  quite  reach  the  equator  at  the 
opposite  side.  But  the  arc  is  certainly  more  than  90  degrees, 
and  varies  in  different  eggs. 

So  far  we  have  traced  the  history  of  the  blastopore  from  six 
o'clock  in  the  morning  to  4.15  in  the  afternoon  of  the  same  day. 
Then  a  remarkable  process  begins.  The  blastopore  moves  back 
as  a  whole  in  exactly  the  opposite  direction  until,  at  7.45  in  the 
evening,  it  has  come  back  to  the  point  from  which  it  started  in 
the  morning.  This  reverse  movement  of  the  whole  blastopore 
is  brought  about  by  quite  a  different  process  from  the  first 
movement  of  the  dorsal  lip.  The  wJiole  egg  rotates  around  a 
horizontal  axis.^ 

The  overgrowth  of  the  lower  pole  by  the  dorsal  and  lateral 
lips  of  the  blastopore  has  covered  the  lower  hemisphere  with 
cells  that  do  not  contain  as  much  pigment  as  do  the  cells  that 
lie  around  the  upper  pole,  i.e.  the  original  black  hemisphere. 
Hence  when  the  egg  rotates  as  a  whole  in  the  way  just  re- 
corded, a  somewhat  lighter  area  will  be  carried  into  the  new 
upper  hemisphere,  while  the  original  upper  hemisphere  will 
noAv  come  to  lie  nearly  on  the  lower  side  of  the  egg.  In  the 
lighter  upper  region,  as  we  shall  soon  see,  the  central  nervous 
system  develops.  The  rotation  of  the  whole  egg  appears  to 
take  place  through  180  degrees,  although  it  is  possible  that  the 

1  An  axis  at  right  angles  to  tlie  median  plane  of  the  later  erabrj-o. 


54 


DEVELOPMENT  OF  THE  FROG'S  EGG 


[Cii.  V 


central  nervous  system  may  also  grow  forward  somewhat  so  that 
the  actual  rotation  is  not  great. 

Perhaps  the  whole  process  may  be  made  clearer  by  reference 
to  a  series  of  sections  through  the  egg.  These  are  taken  through 
the  meridian  that  corresponds  to  the  middle  plane  of  the  body ; 
it  therefore  passes  through  the  upper  and  lower  poles  (Fig.  18). 
The  arrows  indicate  the  primary  axis.  The  dorsal  lip  of  the 
blastopore  lias  formed  in  Fig.  18,  B,  and  in  Fig.  18,  C,  D,  the 
migration  of  the  lip  has  gone  further  over  the  lower  hemisphere. 
The  ventral  lip  of  the  blastopore  has  also  formed.  Figure  18, 
D,  corresponds  to  a  stage  in  which  the  blastoporic  circle  is  com- 
pleted. In  Fig.  18,  E,  we  see  that  the  dorsal  lip  has  travelled 
further  over  the  lower  pole  toward  the  ventral  lip.     Finally, 


Fig.  19.  — a.  Diagram  to  illustrate  overgrowth  of  dorsal  lip  of  blastopore.  I-IV  rep- 
resent different  stages.  B.  Diagram  of  cross-section  through  Z-Y  of  A,  to  show 
lateral  lips  of  blastopore,  and  mesoderm  (M),  and  (IW)  inner  wall  of  archeuteric 
pit.    OW.  Outer  wall. 


in  Fig.  18,  F,  the  egg  is  represented  as  having  rotated  as  a 
whole  to  bring  the  embryonic  portion  above. 

The  changes  that  take  place  during  the  closure  of  the  blasto- 
pore are  perhaps  more  clearly  shown  by  the  following  experi- 
ment. By  means  of  a  fine  pointed  needle  it  is  easy  to  puncture 
the  egg  slightly  at  any  given  point.  When  the  outer  surface 
of  the  egg  is  pierced,  there  follows  a  protrusion  of  material  as 
soon  as  the  needle  is  withdrawn.  At  other  times  when  the 
surface  is  only  indented  {not  pierced)  by  the  needle,  there 
follows  a  blunt  protrusion  of  material  and  the  surface  remains 
unbroken.     In  the  latter  case  the  marks  do  not  always  last  as 


Ch.  V]         EARLY   DEVELOPMENT   OF   THE   EMBRYO  55 

long  as  do  those  produced  by  the  first  method,  but  as  less  harm 
is  done  to  the  egg^  one  can  often  get  more  satisfactory  results.^ 

If  when  the  first  trace  of  the  blastopore  appears  on  the  sur- 
face of  the  egg  (Fig.  19,  A),  a  slight  injury  is  made  to  the 
surface  of  the  white  hemisphere  at  the  side  opposite  the  blasto- 
poric  lip,  i.e.  at  a  point  150  degrees  from  the  dorsal  lip  of  the 
blastopore  (Fig.  19,  A,  at  2;),  we  shall  find  that  in  the  course 
of  four  hours  the  blastopore  will  form  a  crescent,  and  that  the 
distance  from  the  dorsal  lip  to  the  point  of  injury  is  much  less 
than  at  first  (Fig.  19,  III).  A  circular  line  of  pigment  in  the 
white  hemisphere  shows  the  line  along  w^hich  the  lateral  and 
posterior  lips  of  the  blastopore  will  appear.  It  will  be  seen  in 
this  case,  that  the  point  of  injury  lies  therefore  outside  of  the 
yolk-plug,  i.e.  posterior  to  the  ventral  blastoporic  lip. 

In  the  course  of  four  hours  more  it  will  be  found  that  the 
circular  blastopore  is  much  smaller  than  before,  and  that  the 
dorsal  lip  now  lies  much  nearer  to  the  point  of  injury  (Fig.  19, 
IV).  The  dorsal  lip  has  travelled  over  more  than  two-thirds  of 
the  original  distance  from  its  starting-point  to  the  point  of 
injury.  By  making  a  new  experiment  with  an  egg  that  has 
reached  this  stage  of  development,  it  will  be  found  that  when 
the  outlines  of  the  blastopore  have  become  sharply  defined, 
the  later  closure  takes  place  at  nearly  an  equal  rate  from  all 
points  of  the  circumference,  perhaps,  however,  still  somewhat 
more  rapidly  from  the  dorsal  lip  backwards. 

Where  the  diameter  of  the  circle  representing  the  outline  of 
the  egg  equals  27  mm.,^  the  distance  between  the  blastopore 
and  the  injury  measures  21  mm.  In  the  first  four  hours  the 
blastopore  moves  through  8  mm.  In  the  next  four  hours  it 
travels  through  7  mm.,  and  is  therefore  now  only  9  mm.  from 
the  point  of  injury.  By  this  time  the  blastopore  is  circular  in 
outline,  and  the  injury  lies  just  outside  (2  mm.)  of  the  circle. 
The  blastopore  now  measures  7  mm.  in  diameter.  Assuming 
that  from  this  time  forward  the  blastopore  grows  together  at  an 
equal  rate  toward  its  centre,  then  the  dorsal  lip  will  pass  over 
about  one-half   of   the  diameter  of  the  blastopore,  or  4   mm. 


1  This  method  can  be  used  only  with  great  caution. 

^  The  numbers  refer  to  the  measurement  of  the  figure,  and  not  to  the  egg  itself. 


56  DEVELOPMENT   OF   THE   FROG'S   EGG  [Cii.  V 

The  dorsal  lip  has  passed  then,  in  all,  through  19  mm.  of  the 
white  area ;  the  ventral  lip  (from  behind,  forward)  through 
3  mm. 

If  the  region  overgrown  by  the  dorsal  lip  be  compared  with 
the  length  of  the  medullary  folds  which  soon  appear  in  the  same 
region  of  the  embryo,  it  will  be  found  that  the  latter,  when  they 
first  appear,  are  somewhat  longer  than  the  region  overgrown. 
If,  however,  we  deduct  from  the  length  of  the  medullary  folds 
the  thickness  of  the  anterior  connective  that  joins  the  right  and 
left  sides  of  the  nerve-plate,  we  shall  find  that  the  remaining 
length  of  the  medullary  folds  corresponds  very  closely  with  the 
length  of  the  region  overgrown.  We  must,  therefore,  conclude 
that  the  anterior  connective  lies  just  in  front  of  the  point  at 
which  the  first  trace  of  the  dorsal  lip  of  the  blastopore  appeared. 

We  have  assumed  the  point  of  injury  to  be  a  fixed  point  and 
the  overgrowth  to  be  due  to  the  progress  of  the  dorsal  lip.  It 
might  equally  well  have  been  assumed  that  the  overgrowth 
was  only  apparent  and  was  produced  by  the  sliding  forward 
of  the  whole  of  the  white  area  beneath  the  dorsal  lip  of  the 
blastopore.  The  end-result  would  be  the  same,  but  the  process 
different.  There  can  be  no  question,  however,  that  the  move- 
ment is  really  due  to  the  progress  of  the  dorsal  lip.  Other 
experiments  where  two  or  more  points  of  the  surface  are 
injured  show  very  conclusively  that  the  movement  is  a  back- 
ward growth  of  the  rim  of  the  blastopore. 

Comparing  the  statements  made  above  with  those  of  Pfliiger, 
it  will  be  found  that  they  differ  in  three  unimportant  respects. 
The  rapidity  of  the  overgrowth  of  the  very  early  stages,  before 
the  complete  establishment  of  the  crescent,  was  not  noted  by 
Pfliiger.  The  distance  travelled  by  the  dorsal  lip,  as  just 
described,  is  somewhat  less  than  that  given  by  Pfliiger. 
Pfliiger  thought  that  the  dorsal  lip  moved  over  about  180 
degrees,  but  added  that  the  amount  of  the  movement  differed 
in  different  individuals,  and  was  probably  between  90  and  180 
degrees.  My  own  results  make  the  region  of  overgrowth 
about  120  degrees.  From  Pfliiger's  figures  we  are  led  to 
believe  that  the  whole  blastopore  after  the  establishment  of 
its  ventral  lip  continues  to  move  somewhat  nearer  to  the 
equator  of  the  side  nearest  to  the  ventral  lip.     If  this  really 


Ch.  V]         EARLY   DEVELOPMENT   OF   THE   EMBRYO  57 

does  take  place,  in  the  way  shown  by  Pfliiger's  figures,^  it  can 
only  be  due  to  a  slight  rotation  of  the  egg  as  a  whole  in  this 
direction,  for  experiments  show  that  the  entire  movement  of 
the  ventral  lip  is  forward,  i.e.  toward  the  dorsal  lip. 

The  yolk-plug  is  finally  withdrawn  into  the  interior  of  the  egg 
and  the  blastopore  remains  as  a  round  or  often  somewhat  elon- 
gated opening.     Its  subsequent  changes  we  shall  follow  later. 

External  Changes  after  the  Closure  of  the 
Blastopore 

Let  us  next  examine  the  changes  that  appear  at  this  time  in 
the  region  that  now  lies  anterior  to  the  blastopore  and  on  the 
upper  surface  of  the  egg.  There  is  much  variation  in  the  early 
stages  of  development  of  the  embryos  of  a  given  species,  and 
in  different  species  the  variations  are  even  greater.  The  dif- 
ferences in  level  of  different  regions  are  the  result  of  move- 
ments of  the  ectoderm.  To  see  these  to  best  advantage,  the 
livin(/  egg  must  be  placed  in  the  direct  sunlight,  and  the  surface 
studied  with  low  powers  of  the  microscope. 

An  embryo  in  which  the  yolk  is  still  exposed  is  shown  in 
Fig.  20,  A.  Passing  forward  from  the  yolk-plug  over  the 
upper  surface  of  the  egg  is  a  broad  groove,  the  so-called 
"primitive  groove."  At  the  anterior  end  of  the  primitive 
groove  is  a  circular  elevation.  On  each  side  of  the  primitive 
groove,  at  IM,  the  inner  medullary  folds  are  seen.  Outside 
of  these  we  find  a  depression,  and  farther  on  each  side,  at 
EM,  the  outer  medullary  folds.  A  sickle-shaped  depression 
lies  just  in  front  of  the  blastopore. 

A  later  stage  of  the  same  embryo  is  shown  in  Fig.  20,  B. 
The  primitive  groove  is  narrower,  the  medullary  folds  are  more 
distinct,  and  anteriorly  a  continuation  of  the  lateral  folds  has 
formed.  This  will  later  be  called  the  head-fold.  Anteriorly 
and  laterally,  there  is  formed  on  each  side  a  lateral  extension 
of  the  medullary  plate,  the  so-called  "sense-plate." 

The  medullary  plates  now  begin  to  roll  in,  producing  a  deep 
furrow,  the  medullary  furrow  with  the  primitive  groove  at  its 

1  Pfluger  ('83),  PI.  II.,  Figs.  4  and  5  (see  Fig.  18,  F). 


58 


DEVELOPMENT  OF  THE  FROG'S  EGG 


[Ch.  V 


bottom  (Fig.  20,  C).  The  lateral  sense-plate  is  split  into  an 
anterior  and  posterior  part  on  each  side.  The  more  anterior 
part  may  still  be  called  the  sense-plate,  SP,  and  the  posterior, 
the  gill-plate,  GP. 


Fig.  20.  — Surface  views  of  young  embryo  of  Raua.  (After  Schultze.)  IM.  Inner 
medullary  plate.  EM.  Outer  medullary  plate.  GP.  Gill-plate.  SP.  Sense-plate. 
BP,  Blastopore. 


A  later  stage  is  shown  in  Fig.  20,  D.     Here  the  sense-plate 
is  found  to  have  extended  laterally  and  forward,  and  the  two 


Ch.  V]         EARLY  DEVELOPMEXT   OF   THE   EMBRYO 


59 


sides  have  met  in  front  of  the  medullary  folds.  The  gill-plate 
is  also  seen,  but  the  outer  medullary  folds  are  no  longer  con- 
spicuous. The  inner  medullary  folds  are  closing  in  to  form 
a  tube.  The  blastopore  is  reduced  to  an  elongated  slit-like 
opening. 

A  still  later  stage  is  drawn  in  Fig.  20,  E.  The  outline  of 
the  whole  egg  is  now  elliptical,  with  the  long  axis  in  the  direc- 
tion of  the  long  axis  of  the  embryo.  The  medullary  folds  are 
also  much  longer,  and  have  approached  each  other  in  the 
middle  line.    A  deep  furrow  lies  between  the  two  halves.    The 


Gp 


Fig.  21.  — Embryos  of  Rana.     (After  Scliultze.)     Gs,  Gs'.  Two  gill-slits.    Gp.  Gill- 
plate.     Sp.  Sense-plate.     S.  Suckers.     Ax.  Anus. 


folds  have  more  nearly  approached  at  the  middle  of  their 
length,  and  are  more  widely  separated  at  the  anterior  and 
posterior  ends.  At  the  posterior  end  the  medullary  folds  are 
overarching  the  small  elongated  blastopore.  The  sense-plates 
and  the  gill-plates  are  distinctly  visible. 

The  medullary  folds  now  fuse  along  their  whole  length, 
leaving,  as  we  shall  see,  a  central  canal,  which  is  the  overarched 
medullary  furrow.     The  elongation  of  the  embryo  continues  as 


60 


DEVELOPMENT  OF  THE  FROG'S  EGG 


[Ch.  Y 


seen  in  Fig.  21,  A.  The  anterior  end  of  the  medullary  tube 
shows  on  each  side  a  lateral  protrusion,  the  eye-bulb.  At  the 
anterior  end  of  the  body  we  can  see  the  sense-plate  and  on 
each  side  the  broad  gill-plate.  Lying  in  the  sense-plate  on 
each  side  is  a  deeply  pigmented  area  which  is  the  forecast  of 
the  "suckers,"  S.  There  is  a  depression  in  the  middle  line, 
in  the  centre  of  the  sense-plate.  This  depression  marks  the 
mouth-depression,  and  indicates  the  point  at  which,  later,  the 
stomodseal  invagination  will  take  place.     (See  Fig.  20,  F.) 


Fig.  22.  —  Development  of  embryo.    Anterior  view,  showing  sense-plate,  nasal  pits, 
stomodaeum,  and  gills.     (After  Ziegler.) 


A  later  stage  is  shown  in  Fig.  21,  B.  The  relation  of  the 
parts  is  much  as  in  the  last  figure.  The  anterior  end  of  the 
medullary  tube  is  larger  than  before,  and  the  protuberances  of 
the  eye-evaginations  are  more  apparent.  In  the  gill-plate  on 
each  side  appears  a  vertical  depression  and  later  another  de- 
pression behind  it,  GS,  GS'.  These  depressions  mark  the 
external  gill-slits.  The  anus  has  shifted  to  a  more  ventral 
position.  The  suckers  have  each  elongated  ventrally,  and  have 
fused  into  a  pigmented  V-shaped  figure.     The  outer  medulhiry 


Ch.  V]  EARLY  DEVELOPMENT  OF   THE  EMBRYO 


61 


plates  take  no  part  in  ths  rolling  in  to  form  the  medullary  tube, 
but  flatten  out  and  seem  to  disappear. 

Ziegler  ('92)  has  made  several  excellent  figures  of  living 
embryos  of  Rana  temporaria  (Figs.  22,  23).  The  first  of  these 
shows  young  embryos  as  seen  from  in  front,  so  that  the  sense- 
plate  is  turned  toward  the  observer  (Fig.  22).  A  longi- 
tudinal groove  appears  in  the  middle  of  the  sense-plate,  and 
subsequently  a  transverse  groove  develops  across  the  sense- 
plate  (Fig.  22,  D,  E).  The  depression  that  later  forms  the 
mouth  lies  at  the  crossing-point  of  the  longitudinal  and  trans- 


FiG.  23. 


Development  of  embryo,  showing  closure  of  blastopore  and  formation  of 
anus.     (After  Ziegler.) 


There  is  present  on  each  side  above  the  mouth 
a  thickened  ridge  that  forms  the  superior  maxillary  process. 
Below  and  behind  the  mouth  a  pair  of  ridges  appear  that  meet 
in  the  middle  line.  These  are  the  sub-maxillary  processes 
which  later  form  the  lower  jaw.  A  pair  of  depressions  of  the 
surface   ectoderm   helow  the  mouth-area  mark  very  early  the 


Q2  DEVELOPMENT   OF   TPIE   FROG'S   EGG  [Ch.  V 

beginning  of  the  "suckers,"  or  adhesive  glands  (Fig.  22,  D). 
The  nasal  pits  appear  above  the  mouth  (Fig.  22,  F). 

The  outlines  of  the  three  brain-vesicles  can  also  be  faintly 
seen  in  surface  view.  A  pair  of  swellings  on  each  side  of  the 
fore-brain  shows  the  position  of  the  eye-evaginations  (Fig.  22, 
D).  In  the  pharyngeal  region  there  first  appears  on  each  side 
a  vertical  ridge,  and  later  another  ridge  parallel  to  and  behind 
the  first  (Fig.  22,  D,  E).  On  these  ridges  gills  appear  as  pro- 
trusions of  the  surface,  and  later  a  third  ridge  and  gill  are 
formed  behind  and  somewhat  beneath  the  others.  Some  time 
after  hatching,  the  gill-slits  break  through  to  the  exterior  be- 
tween the  ridges  or  gill-arches,  and  at  about  the  same  time, 
the  mouth  breaks  through  into  the  cavity  of  the  pharynx. 

In  the  head-region  the  beginnings  of  some  of  the  spinal 
ganglia  may  be  seen,  and  a  series  of  mesodermal  blocks  also 
appear  and  may  be  dimly  seen  from  the  outer  surface.  These 
structures  do  not,  however,  appear  as  distinctly  in  surface  views 
of  the  embryo  of  Rana  temporaria  as  they  do  in  the  embryos 
of  some  other  species. 

Soon  after  the  nerve-tube  has  closed,  the  dorso-posterior  end 
of  the  body  begins  to  extend  backwards  to  form  the  tail  (Fig. 
23,  D,  E).  The  anal  opening  lies  just  behind  and  ventral  to 
this  region  of  posterior  growth.  The  anus  seems  to  shift  to  a 
more  ventral  position  during  the  elongation  of  the  tail.  At 
first  the  tail  is  a  thick  outgrowth  of  the  posterior  end  of  the 
body,  but  as  it  grows  longer  it  flattens  from  side  to  side,  and 
in  later  stages  a  thin  fan-like  border  or  fin  develops  on  its 
upper  and  lower  margin  (Fig.  38). 


CHAPTER   VI 

FORMATION  OF   THE  GERM-LAYERS 

The  period  that  we  are  now  about  to  examine  is  marked  by 
extensive  movements  of  parts  of  the  segmented  Qgg  as  a  result 
of  which  the  organs  are  formed.  During  the  segmentation- 
period  the  cells  retain,  as  we  have  seen,  the  position  in  which 
they  arise,  but  with  the  appearance  of  the  blastopore  a  new 
period  is  initiated  in  which  extensive  movements  of  cells  and 
groups  of  cells  take  place. 

His's  Experiments  with  Elastic  Plates 

His  ('94),  from  his  studies  of  the  behavior  of  elastic  plates, 
has  concluded  that  many  of  the  phenomena  of  the  develop- 
ing embryo  are  the  mechanical  result  of  the  tensions  set  up 
in  the  different  layers.  In  the  embryo  the  shoving,  compres- 
sion, or  extension  is  supposed  to  result  from  the  unequal  growth 
of  different  parts.  When  a  cell-plate  lifts  itself  up  into  a  fold, 
as  a  result  of  more  rapid  growth  in  that  region  than  elscAvhere, 
there  is  present  on  the  concave  side  a  positive  tension  ("Druck- 
spannung")  and  on  the  convex  side  a  negative  tension.  Under 
these  conditions  the  cells  become  conical,  i.e.  they  are  small  on 
the  concave  side  and  broad  on  the  convex  side  of  the  fold. 
Each  embryonic  cell  tends  of  itself  to  become  spherical  and  only 
the  surrounding  conditions,  resulting  from  the  growth  of  sur- 
rounding parts,  determine  the  shape  of  each  cell  at  any  period 
of  development.  His  has  tried  to  explain  many  of  the  changes 
taking  place  in  the  early  embryo  as  the  result  of  this  simple 
folding  principle.  The  inrolling  of  the  medullary  plate,  the 
formation  of  the  eye-outgrowth  from  this  plate,  the  formation 
of  the  mouth-cavity  and  the  gill-slit-folds,  etc.,  are  examples  of 
some  of  these  changes.     His  pointed  out  how  closely  the  forms 

63 


64:  DEVELOPMENT   OF   THE   FROG'S   EGG  [Ch.  VI 

taken  by  many  of  these  structures  in  the  embryo  resemble 
the  folds  that  can  be  produced  mechanically  by  pulling  out  or 
pushing  in  a  thin  elastic  plate  of  rubber.  If  this  interpretation 
is  true,  it  means  that  at  different  periods  in  the  development, 
regions  of  more  rapid  growth  appear,  now  here,  now  there,  and 
as  a  mechanical  result  of  the  conditions  present,  such  structures 
as  the  medullary  folds,  the  eye-outgrowths,  etc.,  are  produced. 
The  cells  change  their  shape  in  response  to  surrounding  con- 
ditions, i.e.  they  do  not  by  their  individual  activity  or  move- 
ment change  their  shape  to  produce  the  successive  changes  of 
the  embryo,  but  the  shape  of  many  cells  is  changed  as  the 
result  of  growth  or  increase  in  mass  of  certain  regions.  For 
instance,  a  cell  becomes  conical  not  through  its  own  initiative, 
but  because  the  surrounding  pressure  forces  it  into  a  conical 
shape. 

The  Formatiox  of  the  Embryo  by  Concrescence 

The   period  of  overgrowth  of   the  blastopore  when  the  so 
called  process  of  gastrulation  is  going  on  has  been  described  in 
Chapter  V.     We  may  now  follow  the  changes  that  take  place 
in  the  interior  of  the  Qgg  during  that  time. 

When  the  dorsal  lip  of  the  blastopore  appears,  the  cells  have 
shown  little  tendency  to  arrange  themselves  into  sheets  or 
layers.  However,  even  when  the  segmentation-cavity  is  covered 
by  a  roof  of  small  cells,  the  cells  of  the  outer  layer  have  begun 
to  flatten  against  one  another  and  to  form  a  thin  layer  of  cells 
over  the  outer  surface  of  the  black  hemisphere.  In  the  lower 
hemisphere  the  larger  white  cells  do  not  show  such  an  arrange- 
ment. In  the  equatorial  region,  where  the  black  and  white 
cells  meet,  a  careful  examination  of  sections  will  show  that 
there  exists  a  more  or  less  defined  ring  of  cells  stretching 
around  the  embryo,  forming  a  broad  zone  (Fig.  15,  D).  The 
inner  cells  of  this  ring  contain  a  good  deal  of  pigment  around 
the  nuclei.  The  yolk-granules  of  these  inner  cells  are  smaller 
than  the  yolk-granules  in  the  large  white  cells  of  the  lower 
hemisphere,  and  the  cells  of  the  ring  seem  to  contain  also  a 
larger  amount  of  clear  protoplasm.  This  inner  zone  of  cells 
passes,  on  the  one  hand,  by  insensible  gradations  into  the  cells 
of  the  outer  surface  of  the  ring  and  internally  it  is  continuous 


Ch.  VI] 


FORMATIOX   OF   THE   GERM-LAYERS 


65 


with  the  inner  region  of  large  yolk-cells.  This  ring  of  cells, 
as  subsequent  development  sJwws^  is  the  beginning  of  the  embryo, 
and  the  ring  itself  is  composed  of  the  material  ivhich  subsequently 
forms  the  central  nervous  system,  the  mesoderm,  the  notochord,  and 
a  part  of  the  endoderm.  An  understanding  of  the  subsequent 
development  depends  on  a  knowledge  of  the  changes  that  take 
place  in  this  ring. 

The  material  of  the  ring  is  intimately  involved  in  the  move- 
ments that  take  place  during  the  overgrowth  of  the  lower 
hemisphere  by  the  lips  of  the  blastopore.  During  this  period, 
we  must  picture  to  ourselves  the  ring  as  rising  up  and  drawing 
together  over  the  lower  white  hemisphere,  so  that  ultimately 
it  leaves  its  equatorial  position  and  its  halves  come  together  to 
form  the  embryo.     (Fig.  24,  A,  B,  C.) 


A  B  C 

Fig.  24.— Diagrams  illustrating  germ-ring  and  concrescence  of  lips  of  blastoimre. 

As  the  dorsal  lip  of  the  blastopore  progresses  over  the  white 
hemisphere,  its  progress  is  due  to  the  movement  and  fusion 
along  a  meridian  of  the  material  of  the  equatorial  ring.  We 
are  to  think  of  the  material  of  the  ring  as  moving  toward  the 
middle  line  from  the  right  and  left  sides  (for  with  the  estab- 
lishment of  the  dorsal  lip  the  ring  becomes  bilateral)  and 
fusing  continuously  in  the  dorsal  lip  (Fig.  24).  The  advance 
of  the  blastopore  is  merely  the  expression  of  the  absorption 
into  its  dorsal  lip  of  the  material  of  the  two  sides  of  the  ring. 
As  soon  as  the  material  from  the  sides  reaches  the  median  line 
in  the  dorsal  lip  of  the  blastopore,  it  remains  stationary  and 
new  material  is  added  behind  that  just  laid  down.  The  mate- 
rial of  the  equatorial  ring  is  thus  carried  into  a  meridian  of 
the  egg.     With  the  disappearance  of  the  yolk-plug  below  the 


66  DEVELOPMENT   OF   THE   FROG'S   EGG  [Cii.  VI 

surface,  the  final  stages  of  overgrowth  are  completed.  The 
ventral  lip  of  the  blastopore  has  moved  somewhat  forward,  as 
previously  explained,  and  this  slight  forward  movement  proba- 
bly takes  place  by  the  growth  toward  the  median  line  of  the 
material  at  the  sides  of  the  ventral  lip. 

There  are  other  changes  closely  bound  up  with  the  preceding 
phenomena  and,  althougli  these  changes  take  place  simultane- 
ously, it  will  be  necessary  first  to  consider  them  separately,  and 
then  to  try  to  combine  them  into  a  single  statement.  The 
changes  involve,  1)  the  formation  of  the  archenteron,  2)  the 
progression  of  the  blastoporic  rim  over  the  lower  hemisphere, 
3)  the  origin  of  the  middle  layer  or  mesoderm. 

The  Formation  of  the  Archenteron 

1)  When  the  dorsal  lip  appears,  certain  cells  pull  away  from 
the  surface,  leaving  their  outer  pigmented  ends  exposed  for  a  time 
(Fig.  15,  D,  Fig.  12,  H).  These  cells  are  near  the  border-line 
between  the  black  and  white  regions,  but  lie  distinctly  amongst 
the  white  cells.  The  next  change  involves  the  sinking  in  be- 
neath the  surface  of  the  region  in  which  these  cells  are  present. 
The  dorsal  lip  of  the  blastopore  now  begins  its  movement  over 
the  lower  hemisphere.  From  the  surface  we  can  see  that  the 
crescent  becomes  longer  and  longer,  the  horns  extending  out- 
wards along  the  black-white  border  but  well  within  the  white. 
The  same  changes  that  took  place  where  the  dorsal  lip  first 
appeared,  now  take  place  also  wherever  the  crescent  extends. 
First  certain  superficial  cells  pull  into  the  interior  of  the  egg 
leaving  only  their  pigmented  ends  at  the  surface,  and  then  this 
area  of  pigment  sinks  below  the  general  surface.  Simultane- 
ously the  edges  of  the  blastopore  roll  over  the  inturned  (invagi- 
nated)  cells.  The  same  changes  also  take  place  at  the  posterior 
or  ventral  lip  of  the  blastopore,  when  the  two  horns  of  the  lat- 
eral lips  have  met  there.  It  is  necessary  to  examine  sections 
that  have  been  cut  in  several  planes  in  order  to  follow  the 
changes  that  take  place  during  the  further  overgroAvth  of  the 
blastopore.  If  we  examine  a  median  longitudinal  (sagittal) 
section  at  the  time  when  the  dorsal  lip  has  just  begun  to  roll 
over,  we  find  (Fig.  25,  A)  that  a  narrow  space  is  left  between 
the  dorsal  lip  and  the  surface  of  the  lower  hemisphere   over 


Ch.  VI] 


FORMATION   OF   THE   GERM-LAYERS 


67 


which  the  dorsal  lip  has  begun  to  roll.  We  find,  at  the  upper 
end  of  this  crevice,  the  pigmented  ends  of  those  cells  that  were 
previously  at  the  surface.  During  later  stages  the  space,  which 
we  may  at  once  speak  of  as  the  archenteron,  becomes  longer, 
due  to  a  further  progression  of  the  dorsal  lip  over  the  white 
hemisphere.  If  the 
section  were  taken 
somewhat  to  one  side 
of  the  median  line, 
the  length  of  the  ar- 
chenteron would  be 
found  to  be  less  than 
in    the    median    line, 


Fig.  25.  —  A  (small  figure  inside  B).  Longitudinal 
section  through  young  embryo.  B.  Cross-section 
of  last.     (After  Schultze.) 


because  the  rolling  in 
has  been  relatively 
less.  If  we  make  a 
section  at  right  angles 
to  the  last  in  the  plane 
Y-Z,  in  Fig.  19,  A, 
we  cut  the  two  horns 
or  ends  of  the  cres- 
cent. The  cavity  on 
each  side  is  just  be- 
ginning, owing  to  the 
smaller  amount  of  closing  in  from  the  sides  of  the  lateral  lips 
of  the  blastopore.     (Fig.  19,  B.) 

A  section  at  right  angles  to  the  last  section  in  the  plane  of 
the  line  in  Fig.  25,  A,  is  shown  in  Fig.  25,  B.  The  archen- 
teron is  seen  in  the  upper  part  of  the  section.  Its  upper  or 
dorsal  wall  is  made  up  of  small  cells,  while  its  floor  is  formed 
of  large  cells  filled  with  yolk.  The  segmentation-cavity  fills 
the  centre  of  the  section. 

During  the  time  when  the  yolk-plug  is  withdrawing  from 
the  surface,  the  segmentation-cavity  becomes  smaller,  owing, 
without  doubt,  to  the  intrusion  of  the  large  yolk-mass  into  its 
interior,  and  finally,  when  the  archenteron  begins  to  open,  the 
segmentation-cavity  is  almost  entirely  obliterated.  The  seg- 
mentation-cavity is  thus  utilized  by  the  embryo,  for  into  this 
cavity  is  pushed  the  yolk-mass  as  the  latter  is  overgrown  by 


68        DEVELOPMENT  OF  THE  FROG'S  EGG     [Ch.  VI 

the  blastopore-lips.  This  statement  does  not  necessarily  imply, 
however,  that  the  segmentation-cavity  was  prepared  especially 
in  view  of  the  subsequent  changes. 

It  will  be  seen  from  the  foregoing  account  that  the  walls  of 
the  archenteron  are  formed  as  the  blastopore  closes  in.  The 
floor  of  the  archenteron  (Fig.  25,  B)  is  nothing  more  than  the 
surface  of  the  lower  white  hemisphere  that  is  overgrown.  The 
origin  of  the  roof  and  sides  of  the  archenteron  is  somewhat  diffi- 
cult to' understand.  We  have  seen  that  around  the  crescent  of 
the  blastopore  certain  cells  have  pulled  in,  leaving  a  depression 
on  the  surface.  It  is  impossible  to  say  just  how  far  the  cells 
that  pull  in  continue  to  be  drawn  inward,  because  simultane- 
ously the  lips  of  the  blastopore  roll  over.  This  brings  us  to 
a  discussion  of  the  second  topic. 

The  Overgrowth  of  the  Blastoporic  Rim 

2)  There  are  at  least  two  ways  in  which  we  may  think  of 
the  closing  in  of  the  lips  of  the  blastopore,  i.e.  there  are  two 
ways,  either  of  which  might  explain  the  covering  of  the  white 
by  the  black  cells.  We  may  think  of  the  free  edge  of  the 
blastopore  as  growing  toward  a  middle  point.  Or  we  may 
imagine  that  the  lateral  and  dorsal  edges  actually  roll  in 
toward  the  middle  line.  The  latter  process  seems  to  be  that 
which  probably  takes  place,  for  Jordan  ('93)  has  seen  the  outer 
dark  cells  actually  rolling  over  and  into  the  archenteron  in  the 
living  Qgg, 

The  dorsal  and  lateral  walls  of  the  archenteron  will  then  be 
formed  in  part,  or  entirely,  from  those  cells  of  the  surface 
that  have  rolled  in  and  have  come  to  lie  beneath  the  surface. 
These  are  the  cells,  therefore,  that  have  been  at  one  time 
situated  at  the  surface  of  the  embryonic  ring,  and  inasmuch 
as  the  advance  of  the  dorsal  lip  takes  place  very  largely  by 
the  fusion  of  the  lateral  lips,  it  follows  that  the  material  for 
the  greater  part  of  the  dorsal  wall  of  the  archenteron  comes 
from  cells  at  one  time  on  the  outer  surface  of  the  Qgg.  I  am 
inclined  to  think  that  at  first  there  is  also  an  actual  in-pulling 
of  cells  along  the  blastoporic  rim  so  that  cells  at  one  time  below 
the  outer  surface  come  also  to  stand,  later,  at  the  sides  of  the 
archenteron,  i.e.  where  the  dorsal  and  ventral  walls  meet. 


Ch.  yi]  FORMATION   OF   THE   GERM-LAYERS  69 

The  Origix  of  the  Mesoderm 

3)  It  is  difficult  to  give  an  account  of  the  method  of  de- 
velopment of  the  mesoderm,  because  there  are  almost  as  many- 
different  descriptions  of  the  process  as  authors  who  have  de- 
scribed it.  I  have  without  hesitation  set  aside  those  accounts 
where  the  author  has  transparently  sought  to  find  his  precon- 
ceived theories  demonstrated  in  his  drawings  of  the  sections  of 
the  embryo.  In  the  second  place,  several  of  the  more  recent 
accounts  have  started  out,  I  think,  with  a  false  conception  of 
the  position  of  the  embryo  on  the  egg  and  its  method  of  for- 
mation, hence  in  these  accounts  the  method  of  the  formation  of 
the  mesoderm  is  likely  to  be  erroneously  described,  although 
in  several  cases  the  actual  draAvings  of  the  sections  have  been, 
I  believe,  accurately  made.  I  have  followed  as  far  as  possible 
those  interpretations  that  are  in  conformity  with  the  experi- 
mental results  relating  to  the  growth  of  the  embryo.  Certain 
abnormal  embryos,  to  be  described  later  (Chapter  VII),  that 
first  appear  as  a  ring  around  the  egg  throw,  I  think,  also  much 
light  on  the  subject. 

The  cells  that  are  to  form  the  mesodermal  layer  are  present 
at  the  time  when  the  dorsal  lip  of  the  blastopore  has  first 
appeared,  and  even  just  prior  to  that  time.  The  innermost 
of  those  cells  forming  the  ring  around  the  egg  are  the  cells 
that  become  the  mesoderm  (Fig.  19,  B).  These  cells  are 
carried  up  to  the  median  dorsal  line  of  the  embryo  by  the 
closure  of  the  blastopore  (Fig.  24,  A,  B,  C).  They  will  then 
be  found  forming  a  layer  or  sheet  of  cells  (Fig.  25,  B)  that 
separates  itself  on  the  outer  side  from  the  thick  layer  of  small 
ectodermal  cells  (that  has  been  simultaneously  lifted  up)  and 
that  is  separated  on  the  inner  surface,  but  not  very  sharply  if 
at  all,  from  the  dorsal  and  dorso-lateral  walls  of  the  archen- 
teron.  A  continuous  sheet  of  tissue  is  formed  in  this  way 
over  the  dorsal  surface  stretching  across  the  middle  line. 
According  to  some  accounts,  the  fusion  of  this  mesoblastic 
sheet  with  the  endoderm  is  much  closer  in  the  mid-dorsal  line 
than  on  each  side.  We  may,  however,  think  of  the  mesoder- 
mal layer  and  endodermal  layer  as  coming  up  together  to  the 
median   line  from   the  sides,  so  that  we  are  to  think  of   the 


70        DEVELOPMENT  OF  THE  FROG'S  EGG     [Ch.  YI 

mesodermal  and  endodermal  cells  as  being  together  from  the 
begmning. 

Different  Accounts  of  the  Origin  of  the  Archenteron 
AND  Mesoderm 

Before  following  further  the  fate  of  these  concentric  coats 
or  layers  of  cells,  the  so-called  "germ-layers,"  we  may  for  a 
moment  examine  some  other  descriptions  that  have  been  given 
as  to  the  method  of  formation  of  the  archenteron  in  the  frog. 
The  most  common  view  of  the  method  of  gastrulation  of  the 
frog  has  been  that  a  process  of  invagination  takes  place  at  the 
dorsal  lip  of  the  blastopore.  This  process  is  supposed  to  be 
brought  about  by  the  drawing  inwards  and  upwards  of  a  fold 
of  the  outer  wall,  so  that  a  blind  sac  forms.  As  this  presses 
forward  into  the  yolk,  the  latter  pushes  before  it  and  fills  up 
the  segmentation-cavity.  At  the  same  time  the  mesoderm  is 
described  as  growing  forward  from  the  region  of  the  blastopore 
over  the  dorsal  surface  of  the  embryo. 

Other  authors  represent,  however,  the  dorso-lateral  edges 
of  the  archenteron  proliferating  cells  along  the  two  sides  to 
form  the  mesoderm,  while  in  the  mid-dorsal  line  a  solid  block 
of  endoderm  cuts  off  to  form  the  notochord.  Hertwig  has  gone 
so  far  as  to  affirm  that  at  the  dorso-lateral  edges  of  the  archen- 
teron there  are  traces  of  a  pair  of  lateral  pouches  along  each 
side,  and  that  these  give  rise  to  the  cells  that  push  in  between 
the  ectoderm  and  endoderm  to  form  the  middle  layer. 

Robinson  and  Assheton  ('91)  assert  that  the  old  account  of 
the  formation  of  the  archenteron  by  invagination  is  entirely 
erroneous,  and  that  the  cavity  of  the  archenteron  owes  its  exist- 
ence to  a  process  of  progressive  splitting  or  separation  of  the 
large  yolk-cells  of  the  lower  hemisphere,  and  that  this  splitting 
extends  up  into  the  yolk  beneath  the  upper  hemisphere.  The 
dorsal  lip  of  the  blastopore  remains  approximately  stationary 
where  it  first  formed,  and  the  anus  develops  around  this  point. ^ 

1  In  a  later  account  Assheton  ('94)  has  much  altered  his  former  view.  He 
describes  only  the  anterior  end  of  the  archenteron  as  formed  as  a  split  amonp;st 
the  endoderm-cells,  while  the  posterior  third  of  the  archenteron  is,  he  thinks,  the 
result  of  the  overgrowth  of  the  dorsal  and  lateral  lips  of  the  blastopore. 


Ch.  YI]  FORMATION   OF  THE   GERM-LAYERS  71 

Both  assumptions  are,  I  think,  erroneous,  as  a  study  of  the 
changes  that  take  place  in  the  dorsal  lip  will  convince  any  one 
who  will  take  the  trouble  to  follow  in  the  living  egg  the 
method  by  which  the  closure  of  the  blastopore  takes  place. 

Later   Development  of  the  Mesoderm  and   Origin  of 
the  notochord 

Schultze  ('88),  who  has  studied  the  formation  of  the  middle 
germ-layer  of  the  frog,  has  given  an  accurate  account  of  the 
condition  of  the  mesoblast  in  the  embryo  during  the  period  of 
overgrowth  of  the  blastopore.  He  has  done  this,  too,  despite 
the  fact  that  he  believes  the  embryo  of  the  frog  to  be  formed 
over  the  upper  or  black  hemispliere  of  the  egg.  This  belief  has 
not,  however,  in  my  opinion,  vitiated  in  any  degree  his  descrip- 
tion of  the  position  of  the  mesoblast  after  its  formation.  I 
have,  therefore,  reproduced  his  figures  in  Fig.  26,  A-E. 

If  a  cross-section  be  made  through  an  embryo  (in  the  plane 
of  the  dark  line  of  Fig.  25,  A)  at  the  time  when  the  blastopore 
has  assumed  a  crescentic  shape,  we  find  over  the  surface  of 
the  section  a  thick  envelope  of  ectoderm.  The  ectoderm  is  at 
this  time  composed  of  about  four  layers  of  cells  (Fig.  25,  B). 
In  the  outermost  layer  the  cells  are  columnar  in  shape.  In 
the  centre  of  the  section  there  is  a  large  segmentation-cavity 
surrounded  by  large  yolk-bearing  cells.  The  archenteron,  as 
seen  in  cross-section,  is  a  large,  arched  cavity,  its  lower  wall 
formed  by  yolk- cells  and  its  dorsal  wall,  covered  by  a  layer  of 
small  cells  showing  a  tendency  to  become  flattened  against  one 
another.  Above  the  upper  wall  of  the  archenteron,  and  between 
it  and  the  ectoderm,  is  a  thick  layer  of  cells.  This  layer 
stretches  out  on  each  side  of  the  embryo  as  a  lateral  sheet,  but 
the  edges  of  the  sheet  merge  insensibly  into  the  yolk-bearing 
cells  at  the  sides.  Where  this  middle  layer  (mesoderm)  is 
sharply  defined,  we  can  easily  distinguish  its  cells  from  those 
of  the  endoderm,  for  the  mesodermal  cells  are  smaller  and  pig- 
mented. At  the  free  edge  of  the  sheet  it  becomes,  however, 
impossible  to  distinguish  between  the  cells  of  the  mesoderm 
and  of  the  endoderm. 

If   we   examine    a  complete  series  of  sections  through  this 


72 


DEVELOPMENT  OF   THE   FROG'S   EGG  [Cii.  VI 


embryo,  we  find  that  the  hiyer  of  mesoderm  is  inserted  be- 
tween ectoderm  and  yolk-cells  over  all  the  posterior  half  of 
the  embryo.  There  is  a  small  antero-ventral  region  into 
which  the  mesoderm  does  not  extend.  At  a  point  posterior  to 
the  section  described  above,  we  find  the  mesoderm  extending 
mnch  farther  ventrally,  so  as  to  nearly  encircle  this  region  of 
the  embryo.  The  blastopore  is  completely  encircled  by  the 
sheet  of  mesoderm. 


Fuj.  2().  —  A.   Lonjiitiuiimil  section  throuuh  a  younu  embryo  of  Kaua.     B.  C.  D, 
E.  Oross-seetions  of  last  in  planes  of  lines  in  A. 


Cross-sections  through  an  older  embryo  are  drawn  in  Fig.  2G, 
B,  C,  1),  E.  The  tMubryo  has  flattened  along  the  mid-dorsal  line. 
The  ectoderm  has  become  tliinner  along  this  line,  Avhere  a  faint 
groove  can  bo  seen  on  the  surface  of  the  living  egg, —  the  primi- 
tive groove.  On  each  side  of  the  mid-dorsal  line,  the  ectoderm 
is  somewhat  thicker  than  before,  and  the  cells  are  more  closely 
packed  together.  The  ectoderm  over  the  surface  of  the  embryo 
consists  of  an  outer  layer  and  of  several  inner  layers  of  cells. 
The  cavity  of  tlu^  archenteron  has  opened  out  and  is  very  large. 


Ch.  YI]  FORMATIOX  OF   THE   GERM-LAYERS  73 

As  before,  its  ventral  wall  is  composed  of  larger  and  yolk-bear- 
ing cells.  Above  and  laterally  the  walls  are  formed  of  smaller 
cells.  The  latter  have  now  arranged  themselves  in  a  definite 
layer,  and  have  become  somewhat  flattened  (Fig.  26,  B,  C,  D). 
This  layer  is  also  sharply  separated  from  the  mesoderm.  The 
mesoderm,  as  compared  with  its  previous  condition,  has  under- 
gone important  changes.  It  has  extended  further  ventrall}^ 
and  has  met  from  the  right  and  left  sides  in  the  mid-ventral  line 
along  most  of  the  ventral  surface.  Over  the  dorsal  and  dorso- 
lateral walls  of  the  archenteron  it  forms  a  thinner  layer  of  cells 
than  in  the  earlier  embryo  (Fig.  25,  B). 

There  is  still  a  ventral  region  of  the  embryo  where  the  ecto- 
derm and  the  yolk-cells  are  in  contact,  i,e.  a  region  into  which 
the  mesoderm  has  not  extended  (Fig.  26,  C).  The  medullary 
plate  is  seen  in  cross-section.  It  will  be  noticed  that  the  plate 
is  much  thinner  in  the  mid-dorsal  line  than  at  the  sides.  On 
each  side  the  medullar}^  plates  show  a  differentiation  into  two 
parts.  The  most  lateral  and  ventral  edge  of  the  plate  is  formed 
of  cells  less  closely  held  together  than  those  nearer  the  mid- 
dorsal  line.  This  mass  of  rounded  cells  is  the  beginning  of  the 
neural  crest. 

The  mesoderm  in  the  mid-dorsal  line  is  thickened  in  the 
posterior  sections.  According  to  some  writers,  this  median 
mesoderm  has  always  up  to  this  time  remained  closely  fused  with 
the  layer  of  endoderm  beneath  it.  It  marks  the  beginning  of  the 
notoehord. 

The  formation  of  the  notoehord  takes  place  from  behind 
forwards,  so  that  in  the  same  embryo  different  stages  of  its 
development  may  be  found  (Fig.  26,  D,  E). 

Tlie  account  given  above  of  the  formation  of  the  notoehord 
is  not  generall}'  accepted,  particularl}'  since  the  formation  of 
the  notoehord  from  the  endoderm  is  the  method  followed  by 
many,  perhaps  by  all  other  vertebrates.  That  a  median  mass 
of  tissue  stretches  at  first  across  the  dorsal  median  wall  of  the 
archenteron  in  the  frog  cannot  be  denied,  but  many  embrvolo- 
gists  have  preferred  an  interpretation  different  from  that  which 
I  have  followed.  It  is  affirmed  that  there  is  always  a  closer  con- 
nection  between  the  endoderm  and  the  tissue  h'ing  above  it  in  the 
dorsal  median  line  than  between  the  endoderm  on  each  side  of 


74        DEVELOPMENT  OF  THE  FROG'S  EGG     [Ch.  YI 

the  mid-dorsal  line  and  the  mesoderm.  Further,  it  is  said,  that 
the  cord  of  cells  in  the  median  dorsal  line  remains  for  a  longer 
time  connected  with  the  mid-dorsal  endoderm  than  does  the 
mesoderm  at  each  side  with  the  lateral  endoderm,  and  that  the 
notochord  separates  from  its  lateral  connections  (right  and 
left)  with  the  mesoderm,  while  it  still  remains  for  a  time 
closely  fused  in  the  mid-line  with  the  endoderm. 

In  the  newt  and  in  other  urodeles  the  endoderm  in  the  mid- 
dorsal  line  thickens  and  bends  upward  to  form  a  longitudinal 
fold.  The  fold  pinches  off  from  the  endoderm  and  forms  a  cord 
of  cells,  —  the  notochord.  In  the  posterior  end  of  the  toad's 
notochord  the  same  method  of  development  may  be  seen  some- 
times to  take  place.  1 

With  such  clear  evidence  of  the  method  of  formation  of  the 
notochord  from  endoderm  in  the  newt,  it  is  not  surprising  that 
embryologists  have  attempted  to  interpret  the  changes  that 
take  place  in  the  frog  in  the  same  way.  The  main  difficulty 
arises  from  an  unwillingness  on  their  jDart  to  derive  the  noto- 
chord from  the  so-called  middle  germ-layer,  or  mesoderm.  The 
question  therefore  turns,  for  them^  on  what  they  will  call  the 
middle  layer  in  the  frog,  and  what  not  the  middle  layer. 

Since,  however,  all  the  cells  in  this  region  have  had  a  common 
origin,  the  question  is  perhaps  a  trivial  one ;  for  we  cannot  doubt, 
I  think,  that  had  some  of  the  cells  in  the  middle  line  passed  a 
little  to  one  side  or  the  other  of  the  median  line,  they  would 
have  been  capable  of  becoming  mesoderm,  and,  vice  versd^  had 
some  of  the  lateral  cells  come  to  lie  nearer  to  the  middle  line, 
then  they  would  have  taken  j)art  in  the  formation  of  the 
notochord. 

The  notochord  separates  entirely  from  the  mesoderm  and 
endoderm,  and  becomes  rounded  in  cross-section.  On  each 
side  of  the  notochord  the  mesoderm  becomes  thicker,  as  is 
shown  in  Fig.  42.  The  final  stage  in  the  closure  of  the  med- 
ullary folds  and  the  changes  that  take  place  in  the  mesoderm 
will  be  described  in  a  later  chapter. 

1  Field  ('95). 


CHAPTER   YII 

THE  PRODUCTION  OF  ABNORMAL   EMBRYOS  WITH  SPINA 

BIFIDA 

Embryos  of  the  frog  are  occasionally  found  that  differ 
greatly  from  normal  embryos.  Roux,  in  1888,  first  described 
one  of  these  embryos  and  showed  that  a  knowledge  of  its 
structure  and  method  of  development  helped  very  much  tow- 
ard an  understanding  of  the  processes  that  take  place  in  the 


B 

Fig.  27.  —  Two  embryos  formed  as  rings  arouud  equator  of  egg.    A.  Seen  from  in 
front  (produced  in  salt  solution) .    (Morgan.)     B.  Seen  from  side.     (After  Roux.) 

normal  development.  An  embryo  described  by  Roux  is  shown 
in  Fig.  27,  B.  Around  the  equator  of  the  egg  along  the  zone 
between  the  white  and  black  hemispheres  is  a  thickened  ridge. 
A  careful  examination  shows  that  this  ridge  is  not  uniform  in 
thickness,  but  is  bilateral  in  form.  Each  half  is  somewhat 
thickened  at  one  end,  and  resembles  half  of  the  medullary  plate 
of  the  normal  embryo.  Cross-sections  (Fig.  29,  B)  show  that 
these  ridges  around  the  equator  of  the  egg  are  tlie  two  halves 
of  the  medullary  plate.     Instead,  however,  of  being  in  close 

75 


76 


DEVELOPMENT   OF   THE   FROG'S  EGG  [Ch.  VII 


contact,  the  two  half -plates  are  separated  in  the  middle  by  the 
diameter  of  the  egg,  bnt  at  the  anterior  and  posterior  ends  the 
half-plates  unite  to  form  the  ring.  In  section,  a  cord  of  cells, 
the  notochord,  is  found  beneath  each  half  of  the  medullary  fold ; 
and  between  the  yolk-cells  and  the  ectoderm  there  is  also  found 
a  sheet  of  tissue  representing  the  mesoderm.  Hertwig,  in  1892, 
described  a  large  number  of  these  embryos.  One  is  shown  in 
surface  view  as  seen  from  the  white  pole,  in  Fig.  28,  A.  The 
embryo  is  at  a  later  stage  of  development  than,  that  described 
above.     The  exposed  white  yolk,  turned  toward  the  observer, 


Fig.  28. — Two  "  spiua-bifida  "  embryos.     (After  Hertwig.) 
stage  (different  embryos). 


A.  Earlier,  B.  older 


is  surrounded  by  a  groove,  and  outside  of  the  groove  there  is  a 
bounding  darker  ridge.  In  the  anterior  portion  of  the  white 
is  seen  a  crescent-shaped  depression.  A  cross-section  through 
the  middle  of  the  body  of  an  embryo  similar  to  the  last  is 
shown  in  Fig.  29,  A.  The  exposed  yolk  is  seen  at  Y.  On 
each  side  of  this  there  is  a  depression,  and  beyond  the  depres- 
sion a  thickened  ridge  composed  of  ectoderm  cells.  Each  ridge 
passes  over  on  its  outer  side  into  the  ectoderm  that  covers 
all  the  lower  part  of  the  embryo.     Even  in  their  present  stage 


Ch.  YII]        PRODUCTIOX   OF   ABNORMAL   EMBRYOS  77 

of  development  the  ridges  are  clearly  seen  to  be  the  widely 
separated  halves  of  the  medullary  plate.  Beneath  each  half 
of  the  medullary  plate  there  is  a  cross-section  of  the  notochord, 
and  between  the  yolk-cells,  in  the  centre  of  the  section,  and  the 
ectoderm  covering  the  lower  surface,  there  is  a  thick  sheet  of 
cells  representing  the  mesoderm. 

A  longitudinal  (sagittal)  section  of  the  embryo  drawn  in 
Fig.  28,  A,  is  shoAvn  in  Fig.  29,  C.  The  large  exposure  of  yolk- 
cells  (Y)  in  the  upper  part  of  the  figure  is  very  conspicuous. 
A  deep  and  narrow  depression,  bounded  for  the  most  part  by  a 
distinct  layer  of  yolk-cells,  is  found  near  the  anterior  end. 
This  depression  corresponds  to  the  crescent-shaped  opening 
seen  in  surface  view,  and  is  supposed  to  correspond  to  a  part 
of  the  archenteron  of  the  normal  embryo.^  Ectoderm  covers 
the  lower  (ventral)  surface  of  this  section,  and  at  one  point  the 
cells  are  thickened  to  form  the  adhesive  glands  of  the  larva. 
At  the  posterior  end  of  the  embryo  a  small  depression  is  pres- 
ent, and,  as  later  development  shows,  this  corresponds  to  the 
posterior  portion  of  the  archenteron  of  a  normal  embryo. 

Hertwig  found  that  if  male  and  female  frogs  of  certain 
species  be  separated  and  kept  apart  for  several  weeks,  and  the 
eggs  then  be  artificially  fertilized,  an  abnormal  segmentation 
follows,  and,  although  many  of  the  eggs  die,  among  those  that 
live  a  large  number  show  this  condition  of  spina  bifida. 

In  1893  I  made  a  series  of  experiments  attempting  to  pro- 
duce artificially  embryos  showing  spina  bifida,  and  found  that 
they  could  be  made  by  two  entirely  different  methods.  If  the 
segmented  egg,  before  the  blastopore-lips  appear,  be  placed  in 
water  to  which  .6  per  cent,  of  salt  (NaCl)  has  been  added,  the 
later  development  is  modified.  The  dorsal  lip  of  the  blasto- 
pore appears  in  its  normal  position  but  does  not  continue  to 
extend  over  the  white  hemisphere.  The  corners  of  the  lips 
gradually  extend  around  the  equator  of  the  egg.  A  sharp 
line  or  depression  separates  the  black  and  Avhite  hemispheres, 
and  on  the  black  side  of  the  depression  a  circular  ridge  appears, 
which  marks  the  beginning  of  the  medullary  ring  (Fig.  27,  A). 

Similar  embrA'os  may  also  be  produced  if  the  dorsal  lip  of 

1  Possibly  it  represents  in  part  the  liver-diverticulum. 


78 


DEVELOPMENT   OF  THE   FROG'S   EGG  [Ch.  YII 


the  blastopore  is  injured  with  a  needle  at  the  moment  of  its 
appearance,  or  if  the  yolk-mass  in  front  of  the  dorsal  lip  is 
injured  so  that  the  yolk  protrudes  from  the  general  rounded 
surface  of  the  egg.  The  blastopore  is  thus  prevented  from 
extending  backward,  and  its  material  differentiates,  in  situ, 
along  the  equatorial  line.  The  lateral  lips  tend  to  approach 
the  middle  line  and  to  fuse,  but  the  medullary  folds  may 
apj)ear  before  the  fusion  has  taken  place.     There  is  thus  pro- 


FiG.  29.  —  Cross  (A,  B)  and  longitudinal  (C)  sections  through  an  embryo  with  spina 
bifida.    (After  Hertwig.)    M.  Half  medullary  plate.    N.  Half  notochord.    Y.  Yolk. 


duced  an  embryo  with  an  exposure  of  yolk  in  the  mid-dorsal 
line.  The  exposure  is  more  or  less  extensive,  according  to  the 
extent  of  fusion  anteriorly  of  the  blastopore,  and  to  the  extent 
of  fusion  forwards  of  the  lateral  and  ventral  lips. 

These  embryos  with  spina  bifida  show  that  the  material  for 
the  mid-dorsal  surface  of  the  embryos  appears  first  as  a  ring 
around  the  equator  of  the  egg  or  a  little  below  the  equator. 
If  this  material  is  prevented  from  reaching  the  mid-dorsal 
surface,  it  differentiates  in  situ.  Hence  the  production  of  a 
ring-like  medullary  plate  and  a  double  notochord. 


Ch.  VII]        PRODUCTION   OF   ABNORMAL   EMBRYOS  79 

It  is  important  to  know  definitely  the  origin  of  the  material 
that  forms  the  equatorial  ring.  We  have  seen  that  the  ring 
appears  at  the  same  time  that  the  blasto^^ore-lips  extend  around 
the  equator  of  the  egg.  Does  this  material  also  extend  out 
laterally  from  the  dorsal  lip  of  the  blastopore  along  the  sides, 
or  is  the  material  already  present  as  a  circular  ring  of  tissue, 
from  which  the  lips  of  the  blastopore  differentiate  ?  A  study 
of  the  normal  embryo  combined  with  experiments  gives,  I 
believe,  a  conclusive  answer  to  these  questions.  In  the  first 
place,  if  the  dorsal  lip  be  entirely  destroyed,  so  that  it  cannot 
advance,  nevertheless  the  lateral  lips  still  appear  and  extend 
backward.  If  a  point  of  the  surface  be  injured  just  in  front  of 
one  (or  both)  of  the  advancing  corners  of  the  dorso-lateral  lips^ 
the  advance  of  the  latter  would  be  stopped  if  an  actual  transfer 
of  material  were  taking  place;  nevertheless,  on  the  posterior  side 
of  the  point  of  injury,  a  depression  of  the  surface,  marking  the 
blastoporic  rim,  appears,  and  continues  to  extend  backward. 
The  same  thing  happens  if  injuries  be  made  at  two  consecutive 
points  in  the  direction  of  extension  of  the  lateral  lij).  Xow  if 
material  were  actually  transferred  backward  from  the  dorsal 
lip  and  around  the  equator  of  the  egg^  its  movement  Avould  be 
stopped  when  the  dorsal  lip  was  seriously  injured,  so  that  the 
lateral  lips  of  the  blastopore,  and,  later,  the  medullary  folds, 
would  not  appear,  or  else  their  appearance  would  be  delayed. 
Further,  if  there  were,  in  realit}',  any  such  transfer  backward 
of  material  around  the  equator,  its  progress  would  be  stopped 
when  the  material  reached  the  points  of  injury  made  along  the 
line  of  the  lateral  lip.  On  the  contrary,  the  appearance  of  the 
lateral  lips,  after  the  destruction  of  the  dorsal  lip,  takes  place 
as  though  no  hindrance  were  present. 

The  experiments  point  clearly  to  the  conclusion  that  there  is 
no  backward  transfer  of  building  material,  but  that  the  mate- 
rial for  the  dorsal  surface  is  already  present  as  a  ring  around 
or  near  the  equator  of  the  egg. 

If  the  normal  embryo  be  studied  by  means  of  sections  at  the 
period  of  the  extension  of  the  lateral  lips  of  the  blastopore, 
the  material  of  the  ring  is  found  to  be  already  present  in  the 
region  into  which  the  lateral  lips  extend.  The  evidence  from 
these  various  sources  proves  that  the  production  of  the  embryos 


80  DEVELOPMENT   OF    THE   FROG'S   EGG  [Ch.  VII 

showing  spina  bifida  is  owing  to  the  differentiation  in  situ  of  cells 
that  in  the  iiornial  embryo  are  first  carried  to  the  dorsal  surface 
before  they  differentiate  into  their  definitive  organs, 

Roux  first  pointed  out  tliat  tlie  embryo  described  by  him 
showed  that  the  material  for  tlie  two  sides  of  the  embryo  is 
laid  down  in  a  ring,  and  that  by  the  growing  together  (con- 
crescence) of  this  ring  along  the  mid-dorsal  line  of  the  embryo, 
the  two  halves  of  the  body  are  brought  together.  The  same 
method  of  formation  of  the  embryo  by  concrescence  has  been 
described  as  taking  place  in  other  vertebrate  embryos,  and  cer- 
tain writers  have  even  affirmed  that  this  is  the  method  by  which 
all  embryos  of  vertebrates  are  formed.  In  the  main,  Roux's 
conclusion  for  the  frog  seems  to  be  correct,^  but  in  one  respect 
not  an  unimportant  exception  must  be  taken  to  his  statement. 
If  the  material  be  laid  down  as  a  ring  of  tissue  around  the  equa- 
tor, and  if,  by  its  coming  together  (apposition),  the  two  halves 
of  the  embryo  result,  it  follows  that  the  embryo  should  be  at 
least  as  long  as  one  semicircle  of  the  surface  of  the  egg. 
Further,  we  have  seen  that  the  anterior  end  of  the  medullary 
plate  lies  somewhat  above  the  point  of  appearance  of  the  dorsal 
lip  of  the  blastopore,  so  that  the  embryo  would  be,  on  Roux's 
supposition,  even  longer  than  a  semicircle.  But  if  we  measure 
the  medullary  plate  of  the  embryo  at  the  time  of  its  first  appear- 
ance^  we  find  that  in  length  it  is  only  about  one-third  of  the 
length  of  the  circumference  of  the  egg.  It  follows,  then,  that 
as  the  material  comes  to  the  mid-dorsal  line  in  the  normal 
embryo,  it  must  also  become  more  concentrated,  so  that  the 
length  of  the  medullary  plate  is  less  than  the  length  of  the 
material  of  its  halves.  There  is  an  accrescence  or  concentration 
of  material  combined  with  a  concrescence  or  union  of  material 
from  the  two  sides. 


1  Although  Roux  did  not  foresee  the  possibility  that  material  might  grow 
around  the  equator  from  the  dorsal  lip  of  the  blastopore,  my  own  experiments 
show,  I  think,  that  such  a  transfer  does  not  take  place. 


CHAPTER   VIII 

PFLUGER'S  EXPERIMENTS   OX  THE   FROG'S  EGG 

Ix  order  to  discover  how  far  the  development  depends  on 
the  surrounding  conditions  to  which  the  Qg^  is  subjected,  we 
must  change  those  conditions  and  observe  the  result.  In  this 
way  we  may  hope  to  find  out  to  what  extent  the  phenomena  of 
development  are  dependent  on  conditions  outside  of  the  egg^ 
and  how  far  they  result  from  the  Qgg  itself. 

Pfliiger  made,  in  1883,  a  brilliant  series  of  experiments  that 
have  been  the  point  of  departure  for  much  of  the  later  Avork 
on  the  frog's  egg',  therefore,  in  this  chapter,  I  shall  give  a 
somewhat  detailed  account  of  Pfliiger's  work.  The  results 
are  arranged  in  an  order  different  from  that  followed  by 
Pfliiger,  with  the  hope  of  making  clearer  a  necessarily  brief 
abstract. 

The  following  orientation  of  the  Qgg  will  facilitate  the  de- 
scription of  the  experiments.  If  the  middle  ^  point  of  the  black 
hemisphere  of  the  frog's  Qgg  (the  "  black  pole '')  is  imagined 
to  be  connected  with  the  analogous  point  of  tlie  white  hemi- 
sphere (i.e.  with  the  *' white  pole")  by  a  straight  line  passing 
through  the  centre  of  the  egg-,  this  line  forms  the  primary  diam- 
eter ov  primary  axis  of  the  Qgg.  An  imaginar}^  primary  equator 
and  a  system  of  parallels  and  meridians  belong  to  such  a  diam- 
eter. When  the  frog's  Qgg  segments,  the  first  tAvo  cleavage- 
planes  are  found  to  be  vertical  in  Avhatever  position  the  ^gg 
may  lie.  The  line  of  intersection  of  these  first  two  planes 
passes  through  the  centre  of  the  egg^  forming  what  we  may 

^  Pfliiger  does  not  notice  that  in  the  normal  egg  at  rest  this  "  middle  part "  is 
not  necessarih^  the  highest  part  of  the  e^g.  Correspondingh',  the  lower  pole  need 
not  be  the  lowest  point  of  the  ^^g.  For  the  present,  however,  we  must  disregard 
this  distinction. 

G  81 


82  DEVELOPMENT   OF   THE   FROG'S   EGG        [Ch.  VIII 

call  the  cleavage-axis  or  secondary  axis.  To  this  axis  there  also 
belong  an  imaginary  secondary  equator,  parallels,  and  meridians. 
If  the  Qgg  should  be  turned,  after  cleavage^  so  that  neither  the 
primary  nor  the  secondary  axis  is  vertical,  then  the  diameter 
that  stands  at  the  time  vertical  may  be  spoken  of  as  the  tertiary 
axis. 

It  will  be  seen,  from  what  has  been  said,  that  the  imaginary 
primary  and  secondary  axes  (with  their  systems)  turn  with  the 
Qgg^  i.e.  may  be  thought  of  as  constituent  parts  of  the  Qgg\ 
\yhile  the  tertiary  axis  only  corresponds  to  any  diameter  of  the 
Qgg  that  is  for  the  moment  vertical. 

The  Effect  of  Gravity  on  the  Direction  of  the 
Cleavage 

In  normal  eggs  the  first  and  second  cleavages  are  vertical, 
the  third  horizontal.  The  question  arises,  ''Does  there  exist  a 
causal  relation  between  the  cleavage-planes  and  the  egg-axis,  as 
has  always  been  assumed  without  question,  or  do  the  first  two 
cleavages  go  through  the  primary  axis,  only  because  the  latter 
coincides  with  the  force  of  gravity  ?  "  This  can  be  tested  by 
preventing  the  normal  rotation  of  the  egg.,  and  Pfliiger  found  a 
simple  method  by  which  tliis  is  possible. 

When  the  frog's  Qgg  is  removed  from  the  uterus,  it  is  covered 
by  a  thin  coat  of  gelatinous  substance  which  quickly  absorbs 
water  and,  if  sufficient  water  is  present,  a  space  appears  after 
fertilization  between  the  Qgg  and  its  innermost  membrane. 
If  an  Qgg  is  taken  from  the  uterus  and  placed  in  a  dry  watch- 
glass,  and  only  a  drop  of  water  containing  sperm  is  added, 
then  the  membrane  swells  somewliat,  and  sticks  firmly  to  the 
glass ;  if  now  the  right  amount  of  water  is  added,  the  surface 
of  the  Qgg  remains  in  contact  with  the  egg-membranes  and  the 
Qgg  cannot  rotate  as  it  does  under  normal  conditions.  The 
watch-glass  containing  the  Qgg  may  be  turned  in  any  position, 
and  the  Qgg  turns  with  it,  so  that  any  desired  point  of  the  egg's 
surface  may  be  placed  uppermost.  Let  us  imagine  an  Qgg  to 
be  so  turned  that  the  black  pole  lies  on  one  side.  In  the 
course  of  three  hours  the  first  division  comes  in,  but  now  the 
plane  of  the  first  cleavage  may  not  correspond  to  the  primary 


Ch.  VIII]  pflUger's  experiments  83 

axis.  It  follows  always  the  direction  of  the  force  of  gravity, 
i.e.  it  passes  through  the  vertical  diameter  of  the  egg. 

The  second  cleavage  also  is  vertical,  and  its  position  is  also 
determined  by  the  position  of  the  Qgg^  and  by  the  position  of 
the  plane  of  the  first  cleavage.  The  third  cleavage-planes  often 
show  irregularities.  Generally  they  are  at  right  angles  to  the 
first  two,  and  lie  nearer  the  upper  pole  of  the  Qgg^  or,  in  other 
words,  their  position  is  also  influenced  by  the  force  of  gravity, 
for  they  lie  nearer  to  the  pole  that  stands  uppermost  at  the 
time.  It  is  a  remarkable  fact  that  the  subsequent  cleavages 'are 
more  rapid  in  the  upper  than  in  the  lower  hemisphere,  no  matter 
what  region  of  the  e,gg  has  been  placed  uppermost.  Embryos 
develop  from  these  eggs  that  have  been  turned  into  abnormal 
positions,  and  the  embryos  differ  from  normal  embryos  only  in 
the  relative  distribution  of  pigment  over  the  surface  of  the 
body.  Many  have  the  upper  surface  of  the  body  a  light  brown 
color  with  dark  spots ;  others  have  the  head,  the  back,  and 
upper  surface  of  the  tail  almost  free  from  pigment,  and  of  a 
whitish-yellow  color.  The  belly  in  these  embryos  is  more  or 
less  deeply  pigmented.  In  a  few  days,  however,  new  pigment 
develops  over  the  dorsal  surface  of  the  embryo.  It  should  be 
noted  that  these  paler  embryos  often  show  abnormalities,  such 
as  bizarre  excrescences,  irregular  movements,  slower  develop- 
ment, and  that  after  a  few  days  they  begin  to  die. 

Pfliiger  concluded  from  his  experiments  that  an  Qgg  may  be 
divided  in  all  possible  directions  by  the  early  cleavage-planes 
according  to  the  position  in  which  the  experimenter  places  the 
Qgg^  and  from  such  an  egg  a  normal  tadpole  may  develop. 

It  is  not,  however,  entirely  a  matter  of  indifference  what  angle 
is  made  between  the  cleavage-planes  and  the  primary  axes.  It 
is  certain  that  if  the  upturned  hemisphere  contains  more  white 
tlian  black,  a  normal  embr}  o  may  develop ;  but  if  the  upturned 
hemisphere  be  entirely  white,  i.e.  if  the  Qgg  has  been  rotated 
through  180  degrees,  embryos  may  occasionally  develop,  but 
they  are  nearly  always  abnormal  and  soon  die.  It  is  difficult,  in 
fact  almost  impossible,  to  keep  the  white  hemisphere  upward ; 
for  in  nearly  every  case  Pfliiger  found  that  later  a  partial  rota- 
tion of  the  Qgg  took  place,  so  that  a  crescent  of  black  appeared 
above  the  horizon.      One  exceptional  case  is  worth  recording. 


84  DEVELOPMENT   OF   THE   FROG'S   EGG        [Cii.  VIII 

An  egg  was  observed  that  had  its  white  hemisphere  turned 
exactly  upwards  until  the  first  cleavage  came  in.  More  water 
was  then  added,  and  the  egg  retained  its  reversed  position  and 
continued  to  segment  energetically  and  with  wonderful  regu- 
larity. The  upturned  white  hemisphere  was  soon  divided  into 
many  small  cells,  while  the  cells  in  the  lower  black  hemisphere 
were  larger.  A  later  examination  of  the  egg  showed  that  dur- 
ing the  cleavage  the  egg  had  rotated  through  about  45  degrees, 
bringing  a  portion  of  the  black  hemisphere  above  the  horizon. 
Still  later  the  egg  seemed  to  rotate  back  again  into  its  first  re- 
versed position.  After  a  time  the  development  stopped  and  the 
egg  died. 

If,  as  the  preceding  experiments  seem  to  show,  there  exists 
a  relation  between  the  force  of  gravity  and  the  position  of  the 
first  three  cleavage-planes,  it  is  important  to  knoAV  whether 
gravity  acts  only  at  the  moment  of  cleavage  or  whether  the 
action  is  a  slow  and  continuous  one.  Pfliiger  found  that  if 
the  egg  at  the  two-cell  stage  be  rotated  a  few  seconds  before 
the  appearance  of  the  second  furrow,  so  that  a  new  angle  is  made 
by  the  primary  axis  with  the  direction  of  the  force  of  gravity, 
then  the  second  furrow  comes  in  as  though  the  egg  had  not 
been  changed,  and  may  therefore  make  any  possible  angle  with 
the  direction  of  the  force  of  gravity.  The  same  experiment 
can  be  made  if  more  water  is  added  to  an  egg  that  has  already 
segmented  once  in  an  abnormal  position.  The  egg  may  then 
rotate  so  that  the  first  cleavage-plane  is  no  longer  vertical ; 
nevertheless,  the  second  furrow  always  comes  in  at  right  angles 
to  the  plane  of  the  first  furrow,  and,  therefore,  may  make  any 
possible  angle  with  the  direction  of  the  force  of  gravity. 

A  different  result  follows  if  the  egg  has  been  rotated  one 
hour  after  fertilization  and  therefore  some  time  before  the  time 
of  the  first  cleavage.  The  plane  of  cleavage  of  the  second  divi- 
sion is  then  affected,  and  coincides  with  the  direction  of  the 
force  of  gravity.  We  must  conclude  that  an  interval  of  one 
hour  at  least  is  necessary  to  produce  any  change  in  the  egg. 

What  has  just  been  said  with  regard  to  the  second  planes  of 
cleavage  holds  equally  well  for  the  third  cleavage-planes.  If 
the  egg  be  rotated  through  180  degrees  after  it  has  divided 
twice   (into  four  parts),   then  the   third  furrows  come   in   as 


Ch.  VIII]  PFLUGER'S   EXPERniEXTS  85 

though  no  change  had  taken  place,  i.e.  nearer  the  former  upper 
pole.  But  if  the  Qgg  had  been  rotated  one  hour  after  fertiliza- 
tion (or  even  after  the  first  cleavage),  the  third  furrows  would  ap- 
pear on  the  new  upper  hemisphere,  i.e.  nearer  the  present  upper 
pole.  In  this  last  case  the  four  upper  cells  resulting  from  the 
third  division  are  smaller  than  the  lower  four.  This  shows  that 
the  four  upper  cells  of  the  normal  eight-cell  stage  are  smaller, 
not  because  they  are  black,  but,  according  to  Pfliiger,  on  account 
of  their  position  in  relation  to  the  force  of  gravity.  Embryos  de- 
velop from  these  eggs,  but  they  show  many  abnormal  structures. 
Pfliiger  also  rotated  eggs  through  180  degrees  after  the  third 
cleavage  had  come  in.  In  four  hours  and  a  half  the  cells  of  the 
new  upper  hemisphere  were  of  the  same  size  as  those  of  the  new 
lower  hemisphere.  A  normal  egg  at  this  time  would  have 
shown  a  great  difference  in  the  size  of  the  cells  of  the  upper  and 
lower  hemispheres.  It  follows  from  the  last  experiments  that 
gravity  may  affect  not  only  the  first,  second,  and  third  cleavage- 
planes,  but  the  later  stages  as  well.  "  Gravity,"  Pfliiger  said, 
"  according  to  some  unknown  law  regulates  the  cleavage-planes. 
A  simple  explanation  of  the  phenomenon  does  not  seem  possible 
in  the  light  of  the  facts." 

The  Relation  of  the  Plaxes  of  Cleavage  to  the  Axes 
OF  THE  Embryo 

Pfliiger  made  other  experiments  to  determine  whether,  under 
normal  conditions,  there  exists  any  relation  between  the  planes 
of  cleavage  and  the  axes  of  the  embryo.  He  placed  seventeen 
eggs  in  as  many  watch-glasses  and  added  water  containing 
sperm.     The  axes  of  the  eggs  were  vertical. 

The  direction  of  the  plane  of  first  cleavage  was  noted  and 
marked  by  a  line  scratched  on  each  glass.  The  beginning  of 
the  nervous  system  appeared  in  about  forty-eight  hours.  In 
twelve  eggs  the  median  plane  of  the  body  coincided  with  the 
first  cleavage-plane,  or  at  most  the  two  planes  did  not  differ 
more  than  10  degrees.  In  four  eggs  there  was  an  angle  of 
30  to  60  degrees  between  the  two  planes,  and  in  one  egg  one 
of  90  degrees.  Pfliiger  concludes  that  it  is  highly  probable 
from  this  result  that  the  plane  of  the  first  cleavage  and  the 


86  DEVELOPMENT   OF   THE   FROG'S   EGG        [Ch.  VIII 

median  plane  of  the  body  coincide.  The  exceptions  may  be 
due  to  the  rough  treatment  of  the  eggs.^  Newport  ('51)  had 
previously  made  a  similar  experiment  on  normal  eggs,  i.e.  eggs 
not  fixed  artificially,  and  had  reached  the  same  conclusion  as 
Pfliiger,  but  Newport's  results  were  unknown  to  Pfliiger  when 
he  made  his  experiments. 

Pfliiger  was  led  from  certain  results  of  his  experiments  to 
observe  carefully  the  position  of  the  egg  at  the  time  when  the 
normal  embryo  was  developing.  He  found,  as  has  been  already 
described,  that  the  dorsal  lip  of  the  blastopore  appeared  in  the 
white  below  the  equator  of  the  Qgg.  He  noted  in  the  living 
Qgg  that  the  blastopore  slowly  migrated  over  the  white  hemi- 
sphere, and  that  it  finally  closed  nearly  180  degrees  from  the 
point  of  its  first  appearance.  Subsequently  the  whole  egg  slowly 
rotated,  so  that  the  small  blastopore  traced  the  same  path  (but 
in  a  reversed  direction)  over  which  the  dorsal  lip  of  the  blasto- 
pore had  passed.  The  results  show  that  the  nervous  system 
develops  over  the  lower  white  hemisphere  of  the  Qgg.  The 
material  for  the  nervous  system  comes  from  the  substance  of 
the  lips  of  the  blastopore  as  they  move  over  and  cover  the 
lower  hemisphere.  This  material,  from  which  the  nervous 
system  is  formed,  is  at  first  somewhat  lighter  in  color  than 
the  pigmented  hemisphere  of  the  Qgg.  It  is  darker,  however, 
than  the  white  material  of  the  low^er  hemisphere. 

If  in  normal  eggs  the  first  cleavage -plane  corresponds  to  the 
median  plane  of  the  body  of  the  embryo,  does  the  same  relation 
hold  for  eggs  that  have  segmented  in  abnormal  positions  ?  In 
other  words,  does  the  median  plane  of  the  body  in  eggs  that 
have  been  turned  so  that  the  primary  axis  is  no  longer  vertical 
still  correspond  to  one  of  the  primary  meridians  of  the  Qgg.,  or  to 
one  of  the  secondary  (i.e.  segmentation)  meridians?  Pfliiger's 
observations  showed  that  in  eggs  with  oblique  primary  axes 
the  plane  of  the  first  cleavage  is  not  identical  with  the  median 
plane  of  the  embryo,  but  forms  different  angles  with  it.  In 
forty-eight  eggs  there  were  thirty-three  in  which  the  median 
plane  of  the  embryo  coincided  with  the  primary  axis.     In  the 

1  Or  else  to  a  very  early  rotation  of  the  egg,  either  as  it  shifts  around  its  cen- 
tre of  gravity  during  gastrulation,  or  from  the  action  of  surface-cilia.  —  T.  H.  M. 


Ch.  VIII]  pflUger'S  experiments  87 

remaining  eggs  there  were  eight  in  which  the  median  plane  of 
the  embryo  made  angles  between  10  and  25  degrees  with  the 
primary  axis.  In  five  cases  the  angle  was  between  25  and.  45 
degrees,  and  in  two  the  angle  was  as  great  as  45  to  90  degrees. 
Pfliiger  concluded  that  in  abnormally  turned  eggs  the  median 
plane  of  the  embryo  belongs  to  the  system  of  meridians  of  the 
primary  axis  of  the  egg,  —  as  in  normal  eggs ;  and  that  the 
cleavage  of  the  egg  only  breaks  up  the  building  material  into 
small  building  blocks,  and  it  is  of  no  importance  in  the  subse- 
quent stages  of  development  how  the  splitting  up  has  taken 
place. 

Conclusions  from  the  Experiments 

From  the  orientation  of  the  embryo  with  respect  to  the  pri- 
mary axis  in  whatever  position  the  egg  may  be,  it  might  seem 
that  the  material  of  the  egg  is  not  isotropic.  That  is  to  say, 
the  position  of  the  embryo  is  fixed  in  the  egg,  and  the  embryo 
assumes  its  predetermined  position  regardless  of  the  method  of 
segmentation.  A  more  careful  examination  will  show  however, 
Pfliiger  believes,  that  the  egg  is  isotropic. 

It  is  obvious  that  although  in  most  cases  when  the  egg  lies 
with  an  oblique  primary  axis,  the  median  plane  of  the  body 
belongs  to  the  system  of  primary  meridians,  yet  there  are  theo- 
retically an  infinite  number  of  these  meridians  any  one  of  which 
might  happen  to  be  uppermost  and  to  coincide  with  the  median 
plane  of  the  body ;  and  Pfliiger 's  tables  show  that  there  must 
be  a  great  many  possible  primary  meridians  any  one  of  which 
may  become  the  median  plane  of  the  embryo. 

In  the  second  place,  the  dorsal  lip  of  the  blastopore  never 
develops  on  the  upper  hemisphere,  however  the  egg  may  be 
turned.  Pfliiger  says  that,  in  all,  he  has  probably  examined  a 
thousand  eggs,  and  never  once  found  the  blastopore  above.  It 
appears  always  in  the  white  heloiv  the  equator.  Again,  in  an  egg 
abnormally  placed  the  head  always  develops  above  and  the  body 
heloiv.  These  relations  could  not  exist  for  all  positions  of  the 
egg,  if  the  position  of  the  embryo  were  prefixed  in  its  relation 
to  the  primary  meridians. 

Pfliiger  considered  one  other  possibility ;  namely,  that  the 
semi-fluid  contents  of  the  egg  may  rearrange  themselves  in  eggs 


88  DEVELOPMENT   OF   THE   FROG'S   EGG         [Cii.  YIII 

abnormally  turned,  so  that  the  predetermined  material  takes  a 
definite  position,  and  the  blastopore  always  appears  in  its  proper 
hemisphere.  A  rearrangement,  Pfliiger  believed,  does  not  take 
place,  because  the  egg,  if  set  free,  even  after  it  has  been  turned 
for  two  hours,  will  tend  to  rotate  into  its  normal  position.  Such 
an  egg^  set  free  in  its  membrane,  places  the  primary  axis  ver- 
tical, and  this  rotation  will  take  place  even  after  the  first  and 
second  furrows  have  appeared ;  and  this  would  not  be  the  case 
had  there  been  a  rearrangement  of  the  contents. 

Pfliiger  noticed,  however,  in  eggs  that  had  been  turned  into 
abnormal  positions,  that  the  upper,  white  hemisphere  is  often 
darker  in  the  later  stages  than  it  was  at  first,  and  conversely, 
the  black  hemisphere  may  appear  lighter  owing  to  the  loss  of  a 
part  of  its  pigment.  This  is  brought  about,  Pfliiger  believed, 
by  a  streaming  of  the  pigment-granules  of  the  egg,  and  is  not 
a  result  of  the  rotation  of  the  contents  as  a  whole. 

The  position  of  the  dorsal  lip  of  the  blastopore  is  determined, 
then,  in  part  by  the  position  of  the  primary  axis,  and  in  part 
by  the  tertiary  axis,  since  the  blastopore  is  always  in  the  lower 
hemisphere,  however  the  egg  be  turned.  *'  The  primary  axis 
determines  the  meridian,  and  the  tertiary  axis  the  parallel  in 
which  the  dorsal  lip  of  the  blastopore  shall  appear.^'' 

Since  these  statements  are  true  for  all  possible  positions  of 
the  primary  axis,  it  follows  that  all  primary  meridians  are  of 
equal  value.  If  we  think  of  an  egg  with  inclined  primary  axis 
and  imagine  this  egg  rotated  around  such  an  axis,  then  all  the 
primary  meridians  of  the  egg  will  in  turn  come  uppermost. 
Whichever  one  is  brought  to  rest  in  the  vertical  plane,  that  one 
will  symmetrically  halve  the  opening  of  the  blastopore  wlien 
the  latter  develops,  and  on  that  one  the  embryo  will  lie  with  its 
head  turned  upwards.  It  is  this  vertical  meridian  that  coin- 
cides with  the  direction  of  the  force  of  gravity.  In  this 
meridian,  every  part  is  not  of  equal  value,  because  the  blastopore 
appears  only  in  a  certain  region,  and  the  position  of  the  embryo 
is  thus  fixed.  The  appearance  of  the  blastopore  on  the  vertical 
meridian  below  the  equator  marks  the  crystallization-point  of 
the  whole  organization.  In  other  words,  the  egg-substance  has 
at  this  time  one  meridian  polarized.  Pfliiger  says  :  "  I  think  of 
each  half  of  the  egg  after  this  as  polarized,  for  both  halves  are 


Ch.  VIII]  PFLUGER'S   EXPERIMENTS  89 

then  of  equal  value  and  are  composed  of  equivalent  molecular 
roAYS.  Gravity  alone  has  determined  which  of  all  possible 
meridians  shall  be  the  controlling  one." 

He  adds  :  '^  I  imagine  that  the  fertilized  egg  bears  no  more 
relation  to  the  later  organization  of  the  animal  than  the  snow- 
flakes  bear  to  the  size  and  structure  of  the  glacier  that  develops 
from  them.  From  a  germ  there  always  arises  the  same  struct- 
ure because  the  external  circumstances  remain  the  same.  The 
glacier  that  develops  out  of  the  snowflakes  has  always  the  same 
form,  so  long  as  the  external  conditions  are  unchanged." 


CHAPTER   IX 

EXPERIMENTS   OF  BORN  AND   OF  ROUX 

Pfluger,  as  we  have  seen,  believes  that  when  the  frog's  Qgg 
is  rotated  so  that  the  white  hemisphere  is  turned  uppermost, 
no  rotation  of  the  contents  of  the  egg  takes  place.  Born 
('84,  b)  repeated  this  experiment  of  Pfliiger  and  sought,  by 
making  actual  sections  of  the  eggs,  to  tind  out  whether  any 
changes  do  take  place  in  the  interior  of  the  reversed  Qgg.^ 

Sections  through  normal,  fertilized  or  unfertilized  frogs'  eggs 
show  that  there  is  a  peripheral,  darkly  pigmented  rind  in  the 
form  of  a  shell  thickest  at  the  black  pole  (30  to  40  microns) 
and  fading  away  at  the  white  pole  (Fig.  8).  Beneath  the 
black  rind  in  the  upper  hemisphere  lies  a  broAvnish  pig- 
mented protoplasm.  In  the  centre  of  this  and  just  under  the 
black  pole  is  found  in  cross-section  a  clearer  spot  containing 
the  nucleus.  The  yolk  lies  within  the  white  hemisphere.  The 
yolk  appears  coarsely  granular,  while  the  protoplasm  in  the 
dark  hemisphere  is  finely  granular. 

Changes  that  take  place  ix  the  Interior  of  the 
Egg  after  Rotation 

Born  observed  in  the  living  egg  that  when  the  Avhite  hemi- 
sphere is  kept  upward,  it  gradually  becomes  darker  in  color, 
owing  to  the  appearance  of  a  grayish- white  area.  Pfluger  had 
noticed  the  same  phenomenon.  This  area  grows  larger  in  pro- 
portion to  the  length  of  time  that  the  egg  has  been  turned. 

Examination  of  sections  of  an  inverted  Qgg  shows  that  forty- 


1  After  the  first  account  of  Born's  had  appeared,  other  papers  dealing  with 
the  same  subject  by  Roux,  Rauber,  and  O.  Hertwig  were  also  published.  These 
authors  all  agree  with  Born. 

90 


Ch.  IX]       EXPERBIEXTS   OF   BORX  AND  OF   ROUX  91 

five  minutes  after  inversion  a  rearrangement  of  the  contents 
has  begun.  The  heavier  white  yolk  has  begun  to  sink  down 
on  one  side,  taking  the  shortest  path  toward  the  bottom  of 
the  inverted  egg.  As  the  heavier  yolk  sinks  down  in  response 
to  the  action  of  the  force  of  gravity,  the  granular  protoplasm 
rises  up  on  the  opposite  side.  The  two  sorts  of  substances  do 
not  mix  during  the  interchange  of  position,  but  keep  sharply 
separated  from  each  other.  The  pigment-rind  remains  fixed, 
but  loses  something  of  its  thickness.  After  an  interval  of 
forty-five  minutes  to  two  hours,  the  finely  granular  protoplasm 
has  reached  the  highest  point  of  the  egg,  and  has  spread  out 
under  the  surface  of  the  white  hemisphere.  The  yolk  has 
passed  to  the  lower  hemisphere  of  the  inverted  egg,  and  now 
lies  inside  of  the  black  rind. 

This  description  of  the  movement  of  the  contents  of  the  egg 
applies  to  all  those  cases  in  which  the  white  pole  does  not  stand 
exactly  upward,  or,  in  other  words,  where  the  egg  is  turned 
less  than  180  degrees.  When  the  egg  is  completely  inverted 
the  force  of  gravity  causes  the  contents  of  the  egg  to  rearrange 
themselves  in  a  somewhat  different  way.  The  yolk  sinks  down 
on  all  sides,  while  the  lighter  protoplasm  rises  up  through  the 
centre  of  the  egg,  carrying  with  it  the  nucleus. 

Pfliiger  believed  that  eggs  which  had  been  rotated  through 
180  degrees,  and  kept  in  that  position,  did  not  segment  because 
of  the  covering  up  of  a  micropyle,  where  the  black  pole  came 
in  contact  with  the  lower  surface  of  support.  Born  thinks 
that  such  eggs  do  not  segment,  owing  to  the  inability  of  the 
spermatozoa  to  pierce  the  white  rind  which  is  uppermost.^ 

In  the  normal  egg  the  path  of  the  spermatozoon  can  be  fol- 
lowed by  the  trail  of  pigment  passing  in  from  the  surface  of 
the  egg,  which  marks  the  direction  taken  by  the  spermatozoon. 
This  pigment-line  can  also  be  followed  in  the  partially  inverted 
egg,  and  it  is  seen  that  the  male  pronucleus  is  also  carried 
along  in  the  streaming  protoplasm. 

Born  found  in  eggs  that  have  been  partially  inverted,  that 
the  first  cleavage-plane  is  generally  vertical,  passing  through 


1  The  problem  of  the  extrusion  of  the  second  polar  body  in  these  eggs  should 
be  examined. 


92        DEVELOPMENT  OF  THE  FROG'S  EGG     [Ch.  IX 

the  highest  point  of  the  egg.  Sometimes,  however,  the  plane  of 
cleavage  is  oblique  to  the  vertical,  i.e.  occasionally  it  does  not 
pass  through  the  highest  point  of  the  egg.  The  position  of 
this  vertical  or  nearly  vertical  plane  of  cleavage  bears  generally 
some  relation  to  the  path,  or  meridian,  of  streaming  of  the 
contents  of  the  egg.  The  first  plane  of  cleavage  corresponded 
with  the  streaming  meridian  in  about  one-third  of  one  hundred 
recorded  cases.  In  nearly  all  of  the  remaining  two-thirds,  the 
first  cleavage-plane  stood  nearly  at  right  angles  to  the  stream- 
ing meridian.  1 

Born's  results  throw  a  new  and  important  light  on  Pfliiger's 
experiments.  The  force  of  gravity  acts  on  the  rotated  egg 
only  to  bring  about  a  rearrangement  of  the  contents  of  the  egg 
in  accordance  with  the  specific  gravity  of  the  substances  pres- 
ent. This  is  the  only  connection  between  the  direction  of  the 
force  of  gravity  and  the  direction  of  the  planes  of  cleavage. 
We  also  see  why  a  certain  amount  of  time  is  necessary  after 
the  reversal  of  the  egg  for  the  rearrangement  to  take  place. 

The  Cleavage  of  the  Egg  in  a  Centrifugal  Machine 

Roux  ('84)  tested  in  another  way  the  effect  of  gravity  on 
the  segmentation  of  the  frog's  egg.  "What  would  happen," 
he  asked,  "  if  an  egg  were  so  placed  that  at  every  moment  a 
new  point  was  turned  uppermost  ?  Further,  if  gravity  acts 
only  so  as  to  rearrange  the  contents  of  the  egg^  what  would 
take  place  if  a  centrifugal  force  were  applied  to  the  eggs  before 
cleavage  ?  "  Such  a  centrifugal  force  ought  to  cause  the  egg 
to  orient  itself  in  respect  to  the  direction  of  that  force,  in  the 
same  way  that  gravity  causes  the  egg  to  turn. 

A  wheel  rotating  around  a  horizontal  axis  was  used.  To 
this  wheel  were  attached  tin  boxes  into  which  the  eggs  were 
put.  A  box  could  be  placed  at  any  point  along  a  radius  of  the 
wheel.  When  the  machine  made  eighty-four  revolutions  a 
minute,  some  of  the  boxes  were  so  placed  that  the  centrifugal 

1  In  this  case  the  second  cleavage-plane  would  correspond  with  the  meridian 
of  streaming.  Born  states  that  the  median  plane  of  the  embryos,  developing 
from  the  rotated  eggs,  passes  through  the  secondary  meridian  that  cuts  the 
highest  edge  of  the  white  field  in  its  partially  inverted  position. 


Ch.  IX]       EXPERIMEXTS   OF  BORX  AXD   OF   ROUX  93 

force  was  twice  as  great  as  the  force  of  gravity.  When  the 
box  was  at  the  lowermost  point  in  a  revolution,  the  centrifugal 
force  and  the  force  of  gravity  acted  together.  When  the  box 
was  at  the  highest  point  in  its  revolution,  then  the  centrifu- 
gal force  and  the  force  of  gravity  acted  against  each  other. 
With  the  velocity  of  eighty-four  revolutions  a  minute,  the  cen- 
trifugal force  was  greater  than  the  force  of  gravity,  even  at 
the  highest  point  of  a  revolution.  At  the  intermediate  points, 
i.e.  between  the  highest  and  lowest  points,  the  conditions  are 
different  for  each  point,  and  lie  between  the  two  extremes  just 
stated.  Under  the  conditions  of  the  experiment  the  eggs 
rotated  inside  their  membranes,  so  that  the  black  pole  turned 
inward^  i.e.  toward  the  axis  of  rotation,  and  the  white  pole 
turned  outward.  In  other  words,  the  eggs  now  oriented 
themselves  with  regard  to  the  centrifugal  force,  and  not  with 
regard  to  the  force  of  gravity.  Even  when  the  centrifugal 
force  was  only  half  that  given  above,  the  eggs  still  arranged 
themselves  with  reference  to  that  force.  In  the  latter  case  the 
force  of  gravity  was  barely  overcome  by  the  centrifugal  force 
at  the  highest  point  of  the  revolution.  If  a  still  shorter  radius 
were  used,  so  that  at  the  highest  point  of  the  revolution  the 
force  of  gravity  was  three  times  as  strong  as  the  centrifugal 
force,  even  then  the  eggs  oriented  themselves  as  before,  i.e. 
with  the  black  pole  turned  toward  the  axis  of  rotation.  In  all 
of  these  experiments  it  will  be  seen  that  the  centrifugal  force  is 
a  constant  force,  while  the  action  of  gravity  varies  in  direction 
at  each  point  in  the  revolution.  If  a  still  shorter  radius  of  the 
wheel  was  used,  in  which  case  the  centrifugal  force  was  still 
less,  then  the  eggs  retained  any  position  that  they  had  when 
first  put  into  the  box. 

All  of  these  different  possibilities  could  be  realized  at  the 
same  time  by  using  a  series  of  tin  boxes  placed  at  the  proper 
intervals  along  a  radius  of  the  wheel.  "The  apparatus, 
laden  with  ten  to  eighteen  freshly  fertilized  eggs,  was  set  in 
motion.  I  waited  with  great  interest  for  the  appearance  of 
the  first  cleavage.  It  appeared  at  the  normal  time  and  the 
whole  cleavage  proceeded  in  exactly  the  normal  way.  A  nor- 
mal blastopore  appeared,  and  the  formation  of  the  medullary 
folds,  the  brain-folds,  and  closure  of  the  neural  tube,  and  later 


94        DEVELOPMENT  OF  THE  FROG'S  EGG     [Ch.  IX 

the  formation  of  the  suckers,  gills,  and  tail,  all  took  place  nor- 
mally. There  was  no  difference  in  the  time  of  development 
between  the  eggs  in  the  machine  and  the  normal  eggs  outside," 
used  to  check  the  results. 

In  the  eggs  acted  upon  by  the  centrifugal  force,  the  segmen- 
tation-axis corresponded  with  the  egg-axis,  and  showed  no 
relation  to  the  direction  of  the  force  of  gravity.  The  third 
furrows  appeared  nearer  to  the  black  pole,  and  the  black  cells 
always  divided  faster  than  the  white  cells,  regardless  of  the 
position  of  the  egg  in  respect  to  the  force  of  gravity.  The 
blastopore  appeared  in  its  usual  position. 

Roux  concluded  that  Pfliiger's  interpretation  of  his  experi- 
ments in  regard  to  the  action  of  the  force  of  gravity  was  in- 
correct. Roux  said  that  in  his  own  experiment  the  localized 
effect  of  the  force  of  gravity  had  been  done  away  with,  when 
the  eggs  were  slowly  revolved,  i.e.  in  those  eggs  nearest  the  axis 
of  the  machine,  and  still  the  cleavage  appeared  in  these  eggs 
irrespective  of  their  position.  When  the  centrifugal  force  was 
stronger,  it  replaced  the  force  of  gravity,  and  the  eggs  oriented 
themselves  in  regard  to  the  new  force,  and  still  the  cleavage 
and  the  subsequent  development  took  place  normally. 

Roux  j)ointed  out  that  a  possible  objection  might  be  made  as 
to  the  sufficiency  of  his  results.  Since  the  eggs  were  always 
rotated  in  a  constant  plane,  it  might  be  affirmed  that  gravity, 
acting  vertically  in  the  plane  of  rotation,  still  acted  upon  the 
egg.  To  meet  this  objection  a  new  experiment  was  devised. 
Single  eggs  were  placed  in  a  glass  tube  6  cm.  long.  This  tube, 
only  half  filled  with  water,  was  closed  and  fastened  to  the  cen- 
trifugal machine.  During  each  rotation  of  the  apparatus,  the 
eggs  and  the  water  would  fall  from  one  end  of  the  tube  to  the 
other,  so  that  the  orientation  of  the  eggs  would  be  changed 
during  each  revolution.  Nevertheless  embryos  normal  in 
structure  were  produced.  They  were,  however,  small  and 
weak. 


CHAPTER  X 

MODIFICATION  OF  CLEAVAGE  BY  COMPRESSION  OF  THE 

EGG 

Ix  1884  Pfliiger  made  the  important  and  novel  experiment 
of  compressing  the  unsegmented  Qgg  of  the  frog  between  paral- 
lel plates  of  glass.  In  consequence,  the  cleavage  was  modified,- 
and  there  was  found  to  be  a  direct  relation  established  between 
the  planes  of  cleavage  and  the  direction  of  the  pressure  applied. 
The  first  three  planes  of  division  were  at  right  angles  to  the 
compressing  plate.  Pfliiger  explained  these  results  as  due  to 
the  position  Avhich  the  nuclear  spindle  would  take  during  its 
elongation.  The  long  axis  of  the  spindle,  he  thought,  would 
place  itself  in  the  direction  of  least  resistance,  i.e.  in  a  plane 
parallel  to  the  glass  plates ;  and  since  the  division  of  the  cell  is 
at  right  angles  to  the  long  axis  of  the  spindle,  it  will,  therefore, 
be  at  right  angles  to  the  compressing  plates.  Born  ('93)  and 
Hertwig  ('93)  simultaneously  repeated  Pfluger's  experiment, 
making  also  some  modifications  of  the  original  experiment. 
Horn's  account  is  here  followed,  as  it  gives  a  more  detailed 
report  of  the  results. 

Effect  of  Compressixg  the  Segmenting  Egg  betayeen 
Parallel  Plates 

The  eggs  of  Rana  fusca  are  on  an  average  1.5  mm.  in  diame- 
ter. The  distance  between  the  two  glass  plates  in  the  experiment 
was  1.4  mm.,  for  if  less  the  eggs  were  burst  by  the  pressure. 
Since  all  of  the  jelly  around  the  Qg^  was  not  removed,  the  actual 
diameter  of  the  Qgg.,  as  subsequent  measurements  showed,  was 
less  than  the  distance  between  the  tAvo  parallel  plates  (1.4  mm.). 
For  instance,  a  compressed  Qgg  (after  it  had  been  killed  and 

95 


DEVELOPMENT  OF   THE   FROG'S  EGG 


[Ch.  X 


hardened)  ^  measured  in  its  longer  diameter,  parallel  to  the 
two  plates,  1.83  mm.,  while  the  shorter  diameter,  at  right  angles 
to  the  plates,  was  1.2  mm.  The  two  axes,  therefore,  stand  in 
the  relation  of  2:8.  These  figures  apply  to  eggs  compressed 
in  the  direction  of  tiie  egg-axis,  from  above  downward.  When 
the  egg  is  compressed  from  side  to  side  it  will  withstand 
more  pressure.  With  a  distance  of  1.37  mm.  between  the  two 
parallel  plates,  an  egg  compressed  laterally  measured  in  its 
longer  diameter  1.96  mm.,  while  its  shorter  diameter,  from 
plate  to  plate,  measured  0.91  mm.  The  two  axes  therefore 
bear  to  each  other  the  ratio  of  1:2.  The  experiments  may 
be  described  in  detail  under  the  following  categories. 

1)  ^ggs  compressed  m  the  direction  of  the  primary  axis  (Fig.  30, 
A).  The  eggs  taken  from  the  uterus  were  placed  on  a  dry  plate 


B 

Fig.  30.  —  Diagram  showing  three  positions  of  eggs  under  compression. 

of  glass,  SO  that  the  white  pole  was  exactly  downward,  i.e.  the 
egg-axis  was  vertical.  Another  glass  plate  was  then  placed 
over  the  eggs  and  brought  down  into  contact  with  two  sup- 
porting rods,  so  that  the  two  glass  plates  were  1.4  mm.  apart, 
and  the  eggs  correspondingly  compressed.  The  eggs  were  then 
fertilized  and  the  whole  apparatus  put  into  a  dish  of  water. 
The  primary  axis  of  each  egg  was  kept  always  vertical.  When 
the  first  furrow  comes  in,  it  is  vertical,  i.e.  at  right  angles  to 
the  glass  plates,  and  passes  from  the  black  to  the  white  pole 
(Fig.  31,  A),  dividing  the  egg  into  two  symmetrical  halves.  The 
second  furrows  come  in  at  right  angles  to  the  first,  and  are  also 


The  eggs  contracted  very  little  during  the  process  of  hardening. 


Ch.  X] 


MODIFICATION  OF   CLEAVAGE 


97 


vertical,  i.e.  at  right  angles  to  the  glass  plates.  The  second 
furrows  may  cross  the  first  furrow  in  the  middle  of  its  upper 
and  lower  surfaces,  so  that  four  cells  of  equal  size  result ;  or  the 
second  furroAvs  may  sometimes  pass  to  one  side  of  the  middle 
point,  so  that  two  cells  may  be  larger  than  the  other  two  (Fig. 
31,  A).  The  last  result  may  be  due  to  a  slight  obliquity  iii  the 
position  of  the  axis  of  the  compressed  egg.  The  third  furroics 
(which  normally  are  horizontal  and  at  right  angles  to  the  pre- 


3         1 


Fig.  31. — A,  B,  Egg  compressed  axially  (Diagram  A,  Fig.  30).    A.  Above ;  B.  below. 
C,  D.  Egg  compressed  laterally  (Diagram  B,  Fig.  30).    C.  One  side;  D.  other  side. 


ceding  furrows)  are  also  vertical  and  at  right  angles  to  the 
plates,  and  are  generally  parallel  to  the  first  furrow  (Fig.  31, 
A,  B).  The  Qgg  is  now  divided  into  eight  cells,  all  lying  in 
one  horizontal  plane.  In  the  black  hemisphere  the  third  fur- 
rows abut  against  the  second  furrows  (Fig.  31,  A),  but  be- 
low they  as  often  run  into  the  first  furrows,  as  shown  in  the 


98  DEVELOPJNIENT   OF   THE   FROG'S   EGG  [Cii.  X 

figure  (Fig.  31,  B).  The  fourth  furrows  are  also  vertical  (i.e. 
at  right  angles  to  the  plates)  and  generally  run  parallel  to  the 
second  planes  of  cleavage,  as  seen  in  the  figure  (Fig.  31,  A,  B). 
There  is  no  segmentation-cavity  as  yet  present  in  these  com- 
pressed eggs. 

It  is  possible  to  keep  these  eggs  in  position  until  the  blasto- 
pore appears,  and  then  to  follow  its  movements  up  to  a  time 
when  the  medullary  folds  form.  The  blastopore  appears  on 
the  under  side,  i.e.  on  the  white  hemisphere  near  the  edge  of  the 
egg.  It  closes  at  the  opposite  edge  of  the  loAver  surface.  The 
medullary  folds  also  appear  on  the  lower  surface  of  the  Qgg^  and 
remain  there  until  the  embryo  begins  to  lengthen.  The  belly 
is  therefore  turned  upward. 

2)  Eggs  compressed  laterally^  i.e.  at  right  angles  to  the  primary 
axis.,  with  the  black  pole  kept  upward  (Fig.  30,  B).  The  eggs 
were  placed  between  glass  plates  so  that,  when  the  plates  Avere 
turned  vertically,  the  axis  of  the  eggs  also  stood  vertical,  and 
the  compression  was  from  the  sides.  The  first  furrow  is  ver- 
tical and  at  right  angles  to  the  two  glass  plates  (Fig.  31,  C,  D). 
The  furrow  passes  through  the  middle  of  the  Qgg^  dividing  it 
into  two  equal  parts.  Deviations  from  this  mode  of  division 
often  occur.  The  first  division  sometimes  passes  obliquely,  i.e. 
to  one  side  from  above  downward,  but  keeps  always  at  right 
angles  to  the  glass  plates. 

The  second  cleavage  comes  in  also  at  right  angles  to  the 
plates,  and  at  right  angles  to  the  first  furrow,  and  therefore  in 
a  horizontal  position.  It  always  lies  nearer  the  upper  (i.e.  the 
black)  side  of  the  Qgg,  as  shown  in  the  figure  (Fig.  31,  C,  D). 
Two  upper  small  cells  and  two  lower  large  cells  are  formed. 
The  second  furrows  have  come  in  where,  normally,  the  third 
furrows  lie. 

The  furrows  of  the  third  order  appear  first  in  the  upper 
smaller  cells.  They  are  at  right  angles  to  the  glass  plates,  and 
parallel  to  the  first  furrow,  near  to  which  they  often  lie  (Fig. 
31,  C,  D).  Occasionally,  a  furrow  of  the  third  order  may  lie 
parallel  to  the  second,  and  not  to  the  first  furrow ;  it  may  even 
run  along  the  edge  of  the  compressed  Qg^^  and  is  then  parallel 
to  the  compressing  plates.  In  the  lower  cells  the  furrows  of 
the  third  order  also  come  in  vertically  and  at  right  angles  to 


Ch.  X]  MODIFICATION   OF   CLEAVAGE  99 

the  plates.  They  are  generally  more  or  less  parallel  to  the 
first  furrow.  The  furrows  of  the  fourth  order  come  in  as  a 
rule  at  right  angles  to  the  last  furrows,  and  therefore  vary 
in  position  according  to  the  position  of  the  third  furrows 
(Fig.  81,  C). 

The  later  development  of  tliese  eggs  is  as  follows :  if  the  eggs 
have  been  much  compressed,  the  blastopore  appears  always  at 
the  periphery  of  the  flattened  egg^  i.e.  at  the  edge,  and  in  the 
space  between  the  two  plates ;  when  the  eggs  are  not  so  much 
compressed,  the  blastopore  appears  near  the  edge  but  more  or 
less  upon  one  or  the  other  surface.  Curiously  enough,  just 
before  the  closure  of  the  blastopore,  its  opening  is  found  to  lie 
at  the  odgQ  of  the  same  side  at  which  it  first  appeared.  Born  in- 
terprets this  result  as  due  to  a  rotation  of  the  whole  egg  during 
the  closure  of  the  blastopore.  The  eggs,  he  believes,  are  able 
to  rotate  in  the  space  between  the  glass  plates  around  an  axis 
at  right  angles  to  the  plates.  The  medullary  fold  appears  also 
at  the  edge  of  the  compressed  Qgg. 

3)  Eggs  compressed  laterally  and  kept  with  the  black  pole  to 
one  side  (Fig.  30,  C).  If  the  eggs,  laterally  compressed,  are 
kept  after  compression  in  a  horizontal  position,  i.e.  with  the 
primary  axis  horizontal,  other  phenomena  appear.  Under  these 
circumstances,  Born  says  that  a  streaming  of  the  contents  of 
the  egg  takes  place.  The  cleavage  of  these  eggs  corresponds 
in  general  to  that  of  the  laterally  compressed  eggs,  with  nor- 
mall}'  directed,  i.e.  vertical,  axis. 

4)  Eggs  compressed  heticeen  two  plates  oblique  to  each  other^ 
so  that  the  eggs  lie  in  a  ivedge-shaped  space.  The  first  two  fur- 
rows are  at  right  angles  to  the  compressing  plates,  which  are 
inclined  12  degrees  to  each  other.  The  furrows  of  the  third 
order  are  in  the  smaller^  dark  and  more  compressed  cells  at 
right  angles  to  the  plates,  while  in  the  yolk-cells,  which  are  little 
compressed,  the  furrows  are  horizontal.  The  details  of  these 
experiments  of  cleavage  have  not  been  worked  out  b}-  Born  so 
fully  as  in  the  cases  where  the  compressing  plates  were  parallel 
to  each  other. 

Hertwig  ('83,  b)  has  described  the  first  cleavage  of  one  of 
these  eggs  compressed  by  plates  inclined  45  degrees  to  each 
other.     The  first  cleavage  divides  a  smaller  protoplasmic  por- 


100 


DEVELOPMENT   OF   THE   FROG'S   EGG 


[Cii.  X 


tion  from  a  larger  yolk-portion.     It  does  not  therefore  divide 
the  egg^  as  in  the  preceding  cases,  symmetrically. 

Hertwig  found  that  when  eggs  were  compressed  from  above 
downward,  i.e.  flattened  axially  between  parallel  plates,  there 
was  no  agreement  between  the  plane  of  the  first  cleavage  and 
the  median  plane  of  the  embryo.  Four  times  the  two  coin- 
cided, approximately,  with  the  first  furrow,  five  times  with  the 
second,  and  six  times  with  neither.  The  blastopore  closes  in 
these  eggs,  as  Born  had  also  shown,  at  a  point  of  the  white 
hemisphere  opposite  to  that  at  which  it  first  appeared.  In  the 
eggs  compressed  from  the  sides  and  standing  with  the  axis  ver- 
tical, the  blastopore  appeared  generally  at  the  edge  between 
the  two  plates,  and  closed  at  a  point  opposite  to  that  at  which 
it  had  first  appeared. ^  Exceptionally  in  these  eggs  the  blasto- 
pore appeared  on  one  of  the  flattened  surfaces,  i.e.  against  one 
of  the  compressing  glass  plates. 

Effect  of  Compressing  the  Egg  in  a  Glass  Tube 

lioux  has  show^n  that  if  the  frog's  egg  be  sucked  up  into  a 
glass  tube  of  smaller  diameter  than  the  diameter  of  the  egg. 


Fig.  32. 


Segmentation  of  egg  enclosed  in  a  tube.    (After  Hertwig.)    A.  Four-cell 
stage.    B,  C.  Eight-cell  stage,  above  and  below. 


the  egg  will  be  drawn  out  into  a  barrel-shaped  body  and  the 
cleavage  correspondingly  modified.  The  results,  however,  are 
not  always  alike.  This  is  probably  due  to  the  presence  of  a 
large  amount  of  jelly  surrounding  the  eggs,  so  that  they  do 


1  Hertwig  found  that  when  unsegmented  eggs  compressed  between  parallel 
plates  were  rotated  so  that  the  white  pole  was  turned  upward,  the  egg  rotated 


Ch.  X]  MODIFICATIOX  OF   CLEAVAGE  101 

not  always  take  the  shape  of  the  enclosing  tube.  In  order  to 
avoid  this  inconstant  element,  Hertwig  ('93)  repeated  the 
experiment  with  eggs  from  Avhich  much  of  the  jelly  had  been 
cut  away.  The  fertilized  eggs  were  drawn  up  into  cylin- 
drical tubes  in  which  they  assumed  a  short  cylindrical  shape 
(Fig.  32).  The  eggs  lay  with  the  black  hemisphere  against 
one  side  of  the  tube  and  this  side  was  turned  upward,  and  the 
tube  kept  in  a  horizontal  position.  The  first  cleavage  of  such 
an  egg  is  vertical  and  at  right  angles  to  the  long  axis  of  the 
tube  (Fig.  32).  The  second  furrows  are  also  vertical  and  at 
right  angles  to  the  first,  therefore  in  the  direction  of  the  long 
axis  of  the  tube.  The  third  furrows  are  also  vertical  and 
parallel  to  the  first.  The  result  so  far  is  the  same  as  when  the 
eggs  are  compressed  from  above  downward  between  parallel 
glass  plates.  The  fourth  furrows  are  horizontal  and  divide  the 
egg  into  eight  black  and  eight  white  cells. 

Conclusions  from  the  Experlvients 

These  experiments  in  which  the  cleavage  has  been  modified 
by  changing  the  shape  of  the  egg  have  an  important  bearing 
on  the  general  problem  of  cleavage  of  the  egg.  In  the  first 
place,  the  "  induced  "  form  of  the  cleavage  may  give  us  some 
insight  into  the  causes  that  determine  the  direction  of  the 
normal  cleavage-furrows.  In  the  second  place,  we  see  that 
when  an  egg  is  compressed,  the  sequence  of  the  cell-division  is 
very  different  from  the  normal  sequence.  Since  we  get  nor- 
mal embryos  from  eggs  modified  in  this  way,  it  would  seem  to 
follow,  as  Pfliiger  was  the  first  to  point  out,  that  the  cleavage 
simply  divides  the  spherical  egg  into  the  building-blocks  from 
which  the  later  embryo  forms,  and  it  is  a  matter  of  indifference 


as  a  whole  and  tended  to  turn  the  white  hemisphere  downward.  If,  however, 
the  eggs  were  compressed  after  the  two,  four,  or  eight  cell  stage,  they  then  held 
their  position  much  better  when  the  white  pole  was  turned  upward.  If  the 
compression  was  applied  when  the  cleavage  of  the  eggs  had  gone  very  far,  but 
before  the  blastopore  appeared,  it  was  again  found  that  the  rotation  of  the  egg 
as  a  whole  takes  place  (as  in  the  unsegmented  egg).  An  egg  that  has  been 
turned  with  its  white  pole  upward  at  the  two  or  four  cell  stage  and  has  kept  its 
position  during  the  cleavage-period,  no  longer  tends  to  rotate  as  a  whole  during 
the  later  stages  of  cleavage. 


102  DEVELOPMENT   OF   THE   FROG'S  EGG  [Ch.  X 

as  to  the  succession  of  divisions.  The  value  of  this  statement 
will  be  discussed  later. 

These  experiments  show  clearly  that  by  changing  the  form 
of  the  egg^  we  change  at  the  same  time  its  method  of  cleavage. 
Again,  reasoning  from  these  "induced"  forms  back  to  normal 
forms  of  cleavage,  we  see  also  something  of  the  forces  at  work 
there.  Pfliiger  did  not  fail  to  see  the  importance  of  these 
experiments.  He  believed,  as  we  have  seen,  that  the  direction 
of  the  cleavage-planes  results  from  the  direction  of  the  pressure, 
because  when  the  nuclear  spindle  of  the  egg  or  of  a  blastomere 
forms,  the  spindle  elongates  in  the  direction  of  least  resistance, 
that  is,  at  right  angles  to  the  direction  of  the  pressure. 

The  spindle  in  the  egg  axially  compressed  cannot  lie  at 
right  angles  to  the  plates  because  of  the  resistance  of  the  yolk 
below,  but  it  must  elongate  in  a  plane  parallel  to  the  plates. 
Since  the  cleavage  of  the  protoplasm  takes  place  at  right 
angles  to  the  long  axis  of  the  nuclear  spindle,  the  division- 
planes  must  appear  at  right  angles  to  the  plates.  Born  has 
pointed  out  that  this  interpretation  of  Pfliiger  cannot  be  the 
true  one,  because  the  egg  is  not  a  solid  elastic  ball,  but  a  fluid 
globe  with  an  elastic  coat.  The  pressure,  therefore,  will  be 
quickly  equalized  in  all  directions,  and  cannot  act  during  the 
time  of  cleavage  in  any  given  direction. 

Sachs's  law  for  the  direction  of  new  cleavage-planes  seems  to 
apply  to  the  compressed  eggs.  According  to  Sachs,  the  form 
of  the  whole  mass  determines  the  position  of  the  cleavage- 
planes.  Hertwig  refers  the  processes  of  cleavage  directly  to 
the  changes  that  take  place  in  the  nucleus.  He  thinks  that 
the  nucleus  tends  to  assume  the  centre  of  its  sphere  of  activity, 
which  is  the  centre  of  the  protoplasmic  mass.  This  is  not  neces- 
sarily the  centre  of  an  egg  in  which  the  yolk  is  unequally 
distributed.  Hertwig  thinks  that  the  nuclear  spindle  will  then 
elongate  in  the  direction  of  the  greatest  protoplasmic  mass.  If 
we  apply  Hertwig's  hypothesis  to  the  segmenting  frog's  egg^ 
we  see  that  it  appears  to  explain  in  part  the  various  phe- 
nomena. In  the  egg  compressed  in  the  direction  of  its  pri- 
mary axis  and  with  the  primary  axis  vertical  (category  1), 
the  greatest  protoplasmic  mass  will  be,  for  the  first  spindle,  in 
a  horizontal  plane  ;    similarly  for  the  second  spindle.     Hence 


Ch.  X] 


MODIFICATIOX   OF    CLEAVAGE 


103 


the  cleavage-planes  that  come  in  at  right  angles  to  the  cleav- 
age-spindle must  be  vertical  and  at  right  angles  to  the  plates. 
The  third  cleavage-planes  will  be  for  the  same  reason  vertical, 
and  even  the  fourth  planes  may  be  so.  The  number  of  con- 
secutive divisions  at  right  angles  to  the  compressing  plates  must, 
however,  soon  reach  a  limit,  because  the  mass  of  protoplasm 
in  each  cell  will  soon  be  thicker  vertically  than  horizontally. 
When  this  happens,  the  next  cleavage  comes  in  horizontally  or 
parallel  to  the  plates. 

Hertwig's  hypothesis  seems,  therefore,  in  harmony  with  the 
phenomena  of  the  compressed  eggs.  Whether  it  is  of  general 
application  may  be  doubted  because  cases  have  been  recorded 


D' 

D^ 

IrT 

r\\' 

ofeca 

cWJ 

D3 

^^7 

D9 


E  F  G  H 

Fig.  33.  —  Diagrams  to  sliow  the  distribution  of  nuclei  in  compressed  (A-D)  and 
normal  e^g  (E-H).  In  the  upper  series  (A-D)  the  black  hemisphere  is  turned 
toward  the  observer ;  in  the  lower  series  (E-H)  the  egg  is  seen  from  the  side  and 
in  part  from  above  the  black  hemisphere. 

where  the  elongating  spindle  does  not  seem  to  take  the  direction 
of  the  greatest  protoplasmic  mass.  Further,  in  certain  spheri- 
cal eggs  without  yolk,  all  the  axes  are  equal,  and  some  other 
cause  must  be  present  to  determine  the  direction  of  the  spindle. 
Even  in  the  compressed  egg  (category  1)  the  protoplasm  must 
be  radially  symmetrical.  Finally,  it  is  possible  that  the  phe- 
nomena of  the  greatest  protoplasmic  mass  and  the  elongating 
spindle  ma}^  be  only  concomitant  and  not  causal  phenomena,  for 
the  position  assumed  by  the  centrosomes,  which  come  to  lie  at 


104 


DEVELOPMENT   OF   THE   FROG'S   EGG 


[Ch.  X 


the  apices  of  the  spindle,  must  also  be  considered.  The  centro- 
somes  determine  the  position  of  the  poles  of  the  nuclear  spindle. 
Moreover,  the  position  of  apposition  of  the  two  pronuclei  of  the 
egg  may  be  a  further  factor  in  the  first  cleavage. 


The  Distkibution'  of  the  Nuclei  in  the  Compressed 

Egg 

In  the  experiments  recorded  above,  where  the  frog's  egg  is 
compressed  during  the  cleavage-period,  the  distribution  of  nuclei 
in  the  protoplasm  is  different  from  that  in  the  normal  egg. 
This  is  illustrated  in  the  accompanying  diagrams  (Fig.  33). 
Let  us  call  the  segmentation-nucleus  A,  and  its  first  products 


Ch.  X]  MODIFICATIOX  OF   CLEAVAGE  105 

A^-B^.  The  products  of  these  nuclei  we  may  call  B^-B^, 
B^-B*.  The  following  division  will  give  eight  nuclei,  C^-C^, 
and  at  the  sixteen-cell  stage  we  may  call  the  nuclei  C^-D^, 
C^-D^,  etc.,  as  shown  in  the  accompanying  diagram. 

Now  let  us  compare,  using  this  nomenclature,  a  normal  egg 
(Fig.  33,  B)  with  an  axially  compressed  egg  (Fig.  33,  A).  In 
the  normal  egg  at  the  sixteen-cell  stage,  the  nuclei  around  the 
upper  pole  will  be  C^-D\  C^-D^,  C^-D^,  C"-Ds  and  those 
around  the  lower  pole,  C2-D2,  C*-D*,  C^-B^  C^-DS.  On  the 
other  hand,  in  a  compressed  egg  that  has  been  freed  from  the 
compression  after  the  eight-cell  stage,  so  that  the  fourth  fur- 
row has  come  in  horizontally  (Fig.  33,  A-D),  we  find  that  the 
nuclei  in  the  upper  hemisphere  are  C^-C^,  C^-C*,  C^-C^,  C''-C^, 
and  in  the  lower  hemisphere,  DI-D2,  D^-D*,  D^-DS,  D'-D^. 
Thus  there  is  an  entirely  different  distribution  of  the  products 
of  the  nuclear  division  in  the  two  cases,^  yet  normal  embryos 
develop  from  both  eggs. 

The  simplest  and  most  obvious  conclusion  from  this  result  is, 
I  think,  that  the  sequence  of  nuclear  division  during  the  early 
cleavage-period  has  no  relation  to  the  subsequent  formation 
of  the  embryo,  and  that  at  this  time  the  nuclei  are  all  equiva- 
lent. 


1  There  are  several  other  possible  combinations  of  these  sixteen  nuclei,  but  in 
no  case  is  the  distribution  alike  in  the  normal  and  in  the  compressed  egg. 


CHAPTER   XI 

THE  EFFECT  OF  INJURING    ONE    OF   THE   FIRST   TWO 
BLASTOMERES 

We  have  seen  that  the  plane  of  first  cleavage  corresponds  as 
a  rule  with  the  median  plane  of  the  future  embryo,  so  that  one 
of  the  first  two  blastomeres  gives  rise  to  the  cells  that  form  one 
side  of  the  body  of  the  embryo,  and  the  other  blastomere  pro- 
duces the  cells  of  the  other  side.  It  would  seem  then  that 
even  at  the  two-cell  stage  the  axes  of  the  future  embryo  are 
definitely  laid  down.  But  the  most  fundamental  question  re- 
mains unanswered;  viz.,  has  the  Qgg  after  its  first  cleavage 
divided  its  material  into  qualitatively  different  parts  (i.e. 
lias  the  material  of  the  right  side  of  the  body  been  separated 
qualitatively  from  that  of  the  left  side),  or  are  the  first-formed 
blastomeres  still  undifferentiated,  and  their  subsequent  fate 
dependent  on  the  relative  position  they  bear  to  each  other  as 
a  part  of  a  whole  f 

Roux  tried  to  answer  this  question  by  the  following  ingenious 
experiment. 

ROUX'S     EXPEKIMENT     OF     "  KILLING  "     OXE     OF    THE     FiRST 

Two  Blastomeres 

As  soon  as  the  first  furrow  had  passed  through  the  egg^  one 
of  the  resulting  blastomeres  was  pierced  with  a  hot  needle. 
In  order  to  carry  out  the  experiment  successfully,  certain  pre- 
cautions must  be  taken.  The  eggs  as  soon  as  removed  from 
the  uterus  are  scattered  over  a  glass  plate  (under  water)  so 
that  they  lie  singly.  Then  water  containing  spermatozoa  is 
added.  After  ten  minutes  this  water,  clouded  by  the  sperma- 
tozoa, is  poured  off  and  fresh  water  is  added.  When  the  first 
furrow  in  the  eggs  appears,  the  water  is  again  poured  off. 
Each  Qgg  is  held  by  a  pair  of  forceps  and  then  pierced  by  a 

106 


Ch.  XI]         EFFECT   OF   IXJURIXG   A   BLASTOMERE  107 

hot  needle.  The  needle  is  carefully  sharpened,  and  is  re- 
sharpened  after  each  egg  is  operated  upon.  It  is  best  to 
pierce  the  blastomere  in  the  black  hemisphere  near  the  first 
cleavage-plane.  The  needle  passes  through  about  a  half  (or 
more)  of  the  blastomere.  When  the  needle  is  withdrawn,  a 
greater  or  less  amount  of  the  contents  of  the  blastomere  pro- 
trudes where  the  blastomere  has  been  injured.  The  egg  after 
operation  is  returned  to  the  water.  It  is  necessary  to  keep  the 
eggs  under  careful  observation,  because  sometimes  the  blasto- 
mere has  been  only  slightly  injured  and  continues  to  develop 
more  or  less  irregularly.     Such  eggs  should  be  removed. 


A  B 

Fig.  34.  — A.  Hemiembryo  lateralis.    B.  Hemiembryo  anterior.    (After  Roux.) 

Roux  found  that  in  twenty  per  cent,  of  the  eggs  the  unin- 
jured blastomere  lived  and  continued  to  develop.  This  blas- 
tomere by  continued  division  developed  into  a  form  that  may 
be  called  a  "  semimorula  verticalis,"  since  it  is  like  the  vertical 
half  of  a  normal  "morula."  "That  is  to  say,  it  is  a  hemi- 
spherical structure  with  small  deeply  pigmented  cells  above, 
and  with  larger  non-pigmented  cells  below."  The  segmenta- 
tion-cavity is  often  absent;  sometimes  it  is  represented  by  a 
few  loosely  aggregated  cells,  and  sometimes  by  a  cavity  bor- 
dered in  part  by  the  injured  half  of  the  egg  (Fig.  35,  A).  A 
" semiblastula  verticalis"  then  develops  with  a  well-defined 
segmentation-cavity.  A  "  semigastrula  "  stage  is  next  passed 
through.  "  Hemiembryones  laterales "  develop  from  most  of 
these   eggs,  as  seen  in  Fig.  34,  A.       This  figure    shows   that 


108  DEYELOPMEXT   OF   THE   FROG'S  EGG  [Cii.  XI 

the  right  half  of  an  embryo  has  developed  from  the  uninjured 
blastomere.  Half  a  medullary  plate  is  present  along  the  line 
of  separation  of  the  injured  and  uninjured  halves.  Near  the 
posterior  end  of  the  half  plate,  the  yolk  of  the  developed  half 
is  exposed  over  a  small  region  and  surrounded  by  half  of  a 
blastopore  (?).  A  cross-section  of  such  an  embryo  shows 
(Fig.  35,  B)  that  the  half  plate  has  essentially  the  same  form 
as  half  of  the  normal  medullary  plate  ;  that  beneath  this  half 
plate  a  notochord  is  present  forming  a  rod,  round  or  slightly 
oval  in  cross-section;  that  a  small  archenteron  is  present  in 
the  developing  half,  and  that  a  mesodermal  sheet  is  present 
over  the  side  of  the  hemiembryo.  It  is  interesting  to  note 
that  while  only  half  the  medullary  plate  is  present,  yet   the 


B 

Fig.  35.— Cross-sections  through  two  half-embryos  of  different  stages.    (After  Roux.) 

notochord  and  archenteron,  which  are  also  median  structures, 
form  whole  structures  but  of  smaller  size  than  the  correspond- 
ing normal  organs.  Roux  thought  that  the  notochord  was 
very  probably  composed  of  only  half  the  number  of  cells  pres- 
ent in  the  normal  notochord,  but,  OAving  to  a  great  amount  of 
variation  in  the  latter,  it  was  not  possible  to  determine  this 
relation  definitely. 

Pflliger,  Roux,  and  Born  have  shown  that  sometimes  in  the 
normal  development  the  plane  of  the  first  cleavage  corresponds 
to  the  cross-plane  of  the  body  of  the  embryo,  i.e.  the  plane  of 
the  first  cleavage  separates  the  anterior  from  the  posterior  end 
of  the  body.    Under  these  circumstances,  if  one  of  the  first  two 


Ch.  XI]         EFFECT   OF   IXJURIXG  A   BLASTOMERE  109 

blastomeres  had  been  killed,  we  should  have  anticipated,  Roux 
says,  that  ''hemiembrvones  anteriores"  or  "posteriores"  would 
have  appeared.  Roux  claims  that  such  forms  do  really  appear. 
The  same  result  can  be  obtained,  if,  after  the  second  cleavage 
of  the  egg^  two  of  the  four  cells  be  killed,  i.e.  those  two  that  lie 
on  the  same  side  of  the  second  cleavage-plane.  A  hemiembryo 
anterior  (?)  is  shown  in  Fig.  34,  B.  It  has  the  anterior  end 
of  the  medullary  folds  normally  formed,  also  a  normal  chorda, 
mesoderm,  and  archenteron  in  this  anterior  end.  In  every  re- 
spect it  corresponds  to  the  anterior  end  of  a  normal  embryo, 
except  that  the  archenteric  cavity  is  small,  resulting,  Roux 
thinks,  from  the  impossibility  of  pushing  the  yolk-mass  poste- 
riorly, 'as  is  done  in  the  normal  embryo  when  the  archenteron 
enlarges.  Roux  is  uncertain  whether  he  has  seen  any  "  hemi- 
embryones  posteriores,''  although  one  embryo  that  he  found, 
with  thick  and  short  blastoporic  lips,  may  represent  such  a 
form.^  Roux  made  some  further  experiments  in  which  one  of 
the  first  four  blastomeres  was  killed,  and  other  experiments  in 
which  three  of  the  first  four  blastomeres  were  killed.  In  the 
first  case  he  obtained  three-fourth  morulye  and  three-fourth 
blastiiloe;  in  the  latter  case,  one-fourth  blastulae  and  one-fourth 
embryos.  Roux  concluded  from  his  experiments,  "that  the 
development  of  the  frog's  gastrula  and  of  the  embryo  immedi- 
ately following  the  gastrula-stage  is,  after  the  second  cleavage- 
period,  a  mosaic  work  of  at  least  four  vertical  self-developing 
(or  differentiating)  parts."  "How  far  this  mosaic  work  is 
changed  by  a  change  in  the  position  of  material  in  the  later 
development,  cannot  be  determined." 

In  later  stages  in  the  development  of  the  hemiembryos  a  new 
series  of  phenomena  appear,  that  result  in  the  "'reorganization" 

1  We  should  expect,  following  Bonx's  argument,  to  get  as  many  heniiem- 
bryones  posteriores  as  aiiteiiores,  yet  such  does  not  seem  to  be  the  case. 
Hertwig  ('93.  A)  has  maintained  that  it  is  absurd  to  suppose  the  posterior 
end  of  the  blastopore  could  appear  when  there  is  no  anterior  end;  but  this 
supposition  rests,  I  think,  on  an  erroneous  idea  of  the  way  in  which  the 
blastopore  forms,  for  I  have  shown  in  my  experiments  ('94)  that  the  poste- 
rior lips  of  the  blastopore  may  appear  when  the  anterior  lip  has  been  de- 
stroyed. The  experiment  should  be  carefiUly  repeated  with  the  four-cell 
stage,  where  it  is  possible  to  distinguish  the  two  anterior  and  the  two  posterior 
cells. 


110  DEVELOPMENT   OF   THE  FROG'S   EGG  [Ch.  XI 

of  the  half  operated  upon,  and  in  the  subsequent  "postgenera- 
tion" of  the  same. 

Sections  of  eggs  that  have  been  successfully  operated  upon 
show  the  kind  of  change  that  has  taken  place  in  the  injured 
blastomere  as  a  result  of  the  operation.  The  yolk  is  found  much 
vacuolated  in  places,  and  the  protoplasm  in  the  immediate  path 
of  the  needle  has  been  killed,  and  much  changed.  After  a  time 
it  is  found  that  scattered  nuclei  or  nuclear-like  structures  are 
also  present  in  the  injured  half  (Fig.  35,  A).  These  have  come 
from  the  regular  or  irregular  division  of  the  nucleus  of  the 
blastomere  that  has  not  in  most  cases  been  killed  by  the  hot 
needle.  The  developed  half  is  somewhat  larger  than  the  injured 
blastomere,  and  a  sharp  line  of  demarcation  is  at  first  present 
between  the  two  halves.  Even  in  the  early  stages  of  some  eggs 
changes  are  found  to  take  place  that  precede  the  "reorganiza- 
tion "  of  the  injured  half.  Roux  describes  three  sorts  of  re- 
organization-phenomena. The  first  of  these  changes  involves  the 
formation  of  cells  in  the  injured  half.  Nuclei,  surrounded  by 
a  finely  granular  protoplasm,  appear  in  the  injured  blastomere. 
These  nuclei  seem  to  arise  from  two  sources,  —  from  the  nucleus 
of  the  injured  blastomere,  and  from  nuclei  (or  cells)  of  the 
developing  half  that  have  transmigrated.  Around  the  nuclei 
the  yolk  breaks  up  into  cells.  This  cellulation  of  the  yolk  may 
take  place  at  very  different  times.  It  may  be  absent  in  some 
cases  in  a  semigastrula  and  be  present  in  other  cases  in  a  semi- 
morula  or  semiblastula.  The  cellulation  of  the  injured  half 
begins  always  near  the  developing  half,  and  extends  thence 
outward.  The  cells  of  the  injured  half  are  of  various  sizes, 
but  generally  larger  than  the  cells  of  the  uninjured  half. 

The  cellulation  of  the  yolk  takes  place  only  in  the  unchanged 
non-vacuolated  parts.  Where  the  yolk  has  been  much  changed, 
it  is  worked  over  by  another  method,  i.e.  by  the  second  method 
of  reorganization.  These  parts  are  revived  or  reorganized  by 
the  nuclei  or  the  cells  that  have  now  appeared  in  the  injured 
half.  Such  parts  are  either  actually  devoured  by  wandering 
cells  or  slowly  changed  under  the  influence  of  neighboring  cells 
or  nuclei  so  that  they  become  a  part  of  these  cells. 

In  addition  to  the  two  preceding  modes,  a  third  method  of 
reorganization  takes  place.      When  the   yolk   has   been  much 


Ch.  XI]         EFFECT   OF   IXJURIXG  A  BLASTOMERE  m 

injured,  the  surface  may  be  subsequently  covered  by  ectoderm 
that  grows  directly  from  the  developing  half  over  the  injured 
portions.  '-^Postgeneration''''  now  begins  in  the  cellulated  in- 
jured half  and  ultimately  the  missing  half  of  the  embryo  is 
formed.  The  surface  ectoderm  is  first  postgenerated  either  by 
direct  overgrowth  from  the  uninjured  to  the  injured  side,  or 
by  the  formation  of  ectoderm  from  the  cells  of  the  newly  cellu- 
lated yolk.  The  missing  half  of  the  medullar}^  folds  appears 
very  quickly.  Half  a  day  or  a  night  is  often  sufficient  to  change 
a  hemiembryo  lateralis  into  a  whole  embryo  with  a  complete 
medullary  plate.  The  mesoblast  grows  over  to  the  injured 
half,  but  increases  in  length  and  breadth  by  the  addition  of 
new  cells  from  the  cellulated  yolk.  The  formation  of  new 
mesoderm  takes  place  only  along  the  free  edge  of  that  already 
formed.     The  growth  is  in  a  dorso-ventral  direction. 

The  archenteron  is  postgenerated  in  a  way  very  different 
from  the  way  in  which  the  archenteron  of  the  normal  embryo 
is  formed.  The  lacking  half  of  the  archenteron  arises  in 
connection  with  the  half  of  the  archenteron  already  present 
in  the  hemiembryo.  The  yolk-cells  of  the  injured  half  be- 
come radially  arranged  and  a  slit  appears  in  the  postgenerated 
half  extending  out  from  the  archenteron  of  the  hemiembryo. 
The  cells  surrounding  the  slit  arrange  themselves  into  a  lining 
layer  and  the  slit  opens  to  form  the  missing  half  of  the  archen- 
teron. In  general  we  may  say  that  in  the  postgeneration 
of  the  organs  of  the  injured  half,  the  changes  always  proceed 
from  the  already  differentiated  germ-layers  of  the  hemiembryo, 
and  the  postgeneration  takes  place  where  the  exposed  surfaces 
of  the  germ-layers  touch  the  newly  cellulated  yolk-mass  of  the 
injured  half. 

Further  Experiments 

(By  Hertwig,  Endres  and  Walter,  Schultze,  Wetzel,  Morgan) 

We  may  next  consider  the  work  of  others,  who  have,  after 
Roux,  repeated  the  same  experiment  and  made  further  varia- 
tions of  it.  Lastly,  before  a  final  conclusion  can  be  reached  as 
to  the  interpretation  of  the  results,  we  must  carefully  examine 
the  evidence  from  similar  experiments  on  other  forms.      We 


112  DEVELOPMENT   OF   THE   FROG'S   EGG  [Cii.  XI 

shall  be  then  in  a  position  to  understand  more  fully  the  results 
of  the  experiments  on  the  frog's  egg. 

Hertwig  ('93,  b)  was  the  first  to  repeat  Roux's  experiment, 
but  reached  results  diametrically  opposed  to  those  of  Roux. 
At  the  two-cell  stage,  one  of  the  blastomeres  was  stuck  with 
a  hot  needle,^  but  unfortunately  a  detailed  description  of  the 
method  employed  is  not  given  by  Hertwig.  After  the  opera- 
tion ^  the  egg  so  turns  itself  that  the  uninjured  part  rotates 
upward,  while  the  injured  half  is  below.  This  is  owing,  Hert- 
wig says,  to  the  development  of  a  blastula  and  gastrula  cav- 
ity, within  the  uninjured  and  segmented  half.  The  cleavage- 
stages  of  the  egg  are  not  described!  Sections  of  the  blastula 
stage  show  that  in  the  cellulated  half  a  segmentation-cavity, 
having  a  thin  roof,  has  appeared.  This  cavity  lies,  in  the 
present  case,  in  the  centre  of  the  developing  half.  In  other 
embryos,  the  cavity  may  lie  excentrically,  and  in  some  cases  a 
part  of  the  floor  of  the  cavity  may  he  hounded  hy  the  y oik- suh stance 
of  the  undeveloped  half.  Hertwig  interprets  these  results  to 
mean  that  when  one  of  the  first  blastomeres  is  injured,  the 
method  of  development  of  the  other  blastomere  is  very  much 
altered.  The  injured  half  lying  in  contact  with  the  activ^e 
half  plays  only  a  passive  role  in  the  further  development. 

The  injured  blastomere  is  closely  applied  to  the  developing 
half,  and  in  places  passes  continuously  into  the  latter.  Hertwig 
thinks  that  the  yolk  of  the  injured  blastomere  exerts  on  the 
developing  half  an  influence  similar  to  that  which  the  food- 
yolk  of  meroblastic  eggs  exerts  on  the  protoplasmic  portion 
that  forms  the  embryo.  This  injured  yolk-material  comes  to 
lie  in  the  ventral  and  posterior  portion  of  the  embryo. 

Hertwig  ventures  further  to  prophesy  that  if  the  injured 
yolk-mass  had  been  taken  altogether  out  of  the  egg-coat  (i.e. 
from  its  contact  with  the  living  half),  then  there  would  be 
formed  a  normal  embryo  without  defect  and  like  the  normal 
embryo  in  every  respect  except  its  smaller  size. 

It   is  of   importance   to  note  that  Hertwig  describes  other 


1  In  a  few  cases  a  galvanic  stream  was  used  to  kill  the  blastomere. 

2  How  soon  after  is  not  stated. 


Ch.  XI]  EFFECT   OF   INJURING   A   BLASTOMERE  113 

embryos  that  he  obtained  by  Roux's  methods,  and  contrasts 
these  with  those  described  above.  Some  of  the  embryos  showed 
the  condition  of  spina  bifida,  i.e.  with  both  sides  of  the  body 
developed  and  with  a  large  yolk-exposure  in  the  mid-dorsal 
line.^  Others  of  the  embryos  were  only  slightly  injured  by 
the  operation  and  developed  nearly  normally.  In  these  the  en- 
tire dorsal  region  was  well  developed  and  the  blastopore  closed 
to  a  small  ring.  Only  on  the  ventral  side  was  a  small  defect 
found  where  the  outer  and  middle  germ-layers  were  absent. 
In  these  latter  embryos,  and  in  those  showing  spina  bifida, 
Hertwig  believes  the  injured  blastomere  w\as  not  killed  or 
even  sufficiently  injured  to  prevent  its  partial  development. 
That  this  is  the  true  explanation  cannot  be  doubted ;  for  it  is 
not  at  all  unusual  to  find  after  the  operation  that  the  injured 
blastomere  may  separate  off  small  portions  of  itself  as  cells  that 
develop  along  with  the  cells  from  the  uninjured  half.  Here,  it 
seems  to  me,  is  the  uncertain  part  of  Hertwig's  work.  He  has 
not  observed,  as  far  as  stated,  the  segmentation  of  each  egg  on 
which  he  has  operated,  and  consequently  his  results  are  open  to 
the  objection  that  in  many  cases,  where  he  does  not  suspect  it, 
the  injured  cell  has  also  continued  to  divide  and  to  form  a  part 
of  the  later  embryo. 

In  nearly  all  of  the  embryos  described  by  Hertwig  the 
medullary  folds  are  unequally  developed. ^  Hertwig's  attempts 
to  meet  this  fact  do  not  seem  to  me  altogether  satisfactory.  A 
large  number  of  the  embryos  have  developed  unsymmetrically. 
The  ventral  and  posterior  yolk-mass  lies  higher  up  on  one  side 
than  on  the  other.  In  consequence  of  this,  one  side  of  the 
medullary  fold  lies  nearer  to  the  injured  yolk  than  does  the 
other,  and  as  a  result  the  two  sides  of  the  body  are  unevenly 
developed.  The  asymmetrical  position  of  the  blastopore  on 
the  living  part  is  assumed  to  be  the  underlying  cause  of  the 
later  asymmetrical  position  of  the  medullary  folds;  but  for 
the  primary  reason  of  the  lack  of  symmetry  of  the  blastopore 
itself  Hertwig  gives  really  no  explanation,  and  to  state  that  it 


1  Among  these  embryos  Hertwig  describes  one  that  seems  to  have  been  an 
excellent  example  of  Roux's  "hemiembryo  lateralis." 

2  There  are  a  few  exceptions. 

I 


114  DEVELOPMENT   OF   THE   FKOG'S   EGG  [Ch.  XI 

is  due  to  the  "yolk  lying  higher  up  on  one  side"  is  only  begging 
the  question.  Roux  has  not  failed  to  notice  the  incomplete- 
ness of  Hert wig's  explanation,  and  has  interpreted  all  of  Hert- 
wig's  results  as  due  to  a  sudden  postgeneration  of  the  injured 
half  of  the  embryo;  i.e.  Roux  believes  a  half-embryo  to  have 
first  formed,  and  then  to  have  been  quickly  followed  by  an  im- 
perfect formation  of  the  other  half.  Hence  the  asymmetry  of 
the  embryos. 

It  is  impossible  to  sa}^  how  far  postgeneration  has  played 
a  part  in  the  development  of  the  embryos  described  by  Hert- 
wig,  but  that  postgeneration  will  explain  all  the  difference 
between  the  results  of  Roux  and  of  HertAvig  seems  highly 
improbable.  Further,  as  I  have* said,  it  seems  not  unlikely 
that  many  of  the  embryos  described  by  Hert  wig  have  come, 
not  only  from  the  uninjured  blastomere,  but  also  from  a  part 
of  the  injured  blastomere.  If  this  latter  supposition  be  true, 
we  can  better  understand  why  the  injured  yolk  forms  in  many 
cases  an  integral  part  of  the  developing  embryo. 

Hertwig  has  made  a  most  formidable  attack  on  Roux's  expla- 
nation of  postgeneration  of  the  embryo.  The  subject  itself  is  of 
secondary  importance  as  compared  with  the  main  problem  in- 
volved in  the  experiment,  and  yet  of  sufficient  interest  to  war- 
rant careful  examination.  Roux  describes  the  blastomere  into 
which  the  hot  needle  has  been  plunged  as  dead,  and  speaks  of  a 
later  revivification  of  the  dead  half  of  the  Qgg.  Hertwig  be- 
lieves that  all  of  that  part  of  the  operated  blastomere  that  is 
later  divided  up  into  cells  (to  be  used  in  the  development)  is 
not  dead,  but  only  more  or  less  injured.  Only  a  small  portion 
of  the  injured  blastomere  is  really  dead,  and  that  is  the  por- 
tion which  has  become  coagulated  by  the  hot  needle.  This 
portion  cannot  later  be  broken  up  into  cells,  but  may  be  either 
thrown  out  by  the  living  embryo  or  assimilated,  owing  to  the 
power  of  digestion  of  neighboring  cells.  The  injured  blasto- 
mere behaves  in  the  same  way  that  a  portion  of  the  body  of  an 
animal  would  if  a  needle  had  been  stuck  into  it.  The  place 
injured  might  quickly  heal,  and  the  comparatively  small  region 
that  had  been  pierced  and  killed  would  be  reabsorbed  again. 
If  the  needle  had  been  first  heated,  the  region  of  injury  would 
only  be  larger,  and  the  necrotic  tissue  would  be  either  thrown 


Ch.  XI]         EFFECT   OF   IXJURING  A  BLASTO.MERE  II5 

off  or  absorbed.  It  has  been  shoAvn  by  Roux  that  when  a 
blastomere  has  been  pierced  by  a  cold  needle,  there  is  a  small 
outflow  of  yolk,  and  the  injured  blastomere  continues  to  divide 
at  the  same  rate  as  the  uninjured  cell.  When  the  needle  is 
heated,  the  cleavage-process  is  delayed  or  prevented,  while  it 
continues  on  the  uninjured  side ;  but  after  a  time  the  injured 
blastomere  may  also  begin  to  divide  in  an  irregular  way. 
After  two  or  three  days  one  gets  generally  from  such  eggs 
quite  normal  gastrulse  and  embryos,  differing  in  little  or  no 
respect  from  embryos  from  uninjured  eggs. 

The  nucleus  of  the  uninjured  blastomere  may  continue  to 
divide,  although  the  protoplasm,  owing  to  its  injury,  may  not 
be  able  to  do  so  for  some  time.  The  nuclei  may  scatter  them- 
selves through  the  protoplasm  (and  yolk),  and  subsequently 
take  part  in  the  division  of  this  into  cells.  In  extreme  cases 
Hertwig  admits  that  when  the  needle  is  very  hot,  the  whole 
of  the  protoplasm  of  the  blastomere  may  be  killed,  and  also 
the  nucleus.  Furthermore,  it  is  possible  that  occasionally  the 
heat  may  radiate  from  the  one  blastomere  into  the  other  and 
partially  kill  this  other  one  also.  If  the  last  condition  is 
brought  about,  the  development  of  the  partially  injured  blasto- 
mere may  take  place  only  very  slowly,  if  at  all.  In  most  cases, 
therefore,  Hertwig  believes  a  '^  reorganization "  of  the  injured 
cell  takes  place,  and  not  a  ^'  revivification ''  of  the  dead  half. 
In  this  reorganization,  Hertwig  thinks  that  the  nucleus  of  the 
injured  cell  itself  plays  the  main  part,  while  Roux  believed 
the  process  was  brought  about  largely  by  an  immigration  of 
cells  (or  nuclei)  from  the  uninjured  into  the  injured  half. 
Hertwig's  conclusion  here  seems  based  rather  on  a  priori 
probability,  while  Roux's  statements  rest  directly  on  his  own 
observations.  Recently  the  same  ground  has  been  worked  over 
by  Endres  and  Walter,  whose  results  substantiate  Roux  in 
every  respect. 

Endres  and  Walter  (*95)  have  obtained  the  typical  half- 
blastulae  and  half-gastruke  and  half -embryos  which  Roux  has 
described.  They  deny  that  ivhole  embryos  develop  from  one  of 
the  first  two  blastomeres,  as  Hertwig  affirmed.  Their  figures 
show  in  the  most  conclusive  way  that  half-emhryos  do  develop 


116  DEVELOPMENT   OF   THE   FROG'S   EGG  [Ch.  X[ 

under  the  conditions  of  Roux's  experiment.  The  subsequent 
postgeneration  of  the  injured  half  of  the  egg  has  also  been 
studied  by  these  authors.  They  confirm  in  every  detail  the 
method  of  reorganization  and  postgeneration  of  the  injured 
half  as  described  by  Roux.  The  reorganizing  cells  have  migrated 
from  the  uninjured  to  the  injured  side,  and  there  have  caused  the 
protoplasm  to  break  up  into  cells.  The  injured  blastomere  is 
also  overgrown  directly  by  the  ectoderm  of  the  uninjured  and 
developing  side.  In  many  of  these  embryos  the  right  and 
left  side  (one  side  has  postgenerated)  are  separated  from  each 
other  by  a  protruding  yolk-mass,  forming  spina-bifida  embryos. 
The  reorganization  of  the  much  changed  mass  of  the  injured 
blastomere  is  brought  about  by  being  assimilated  by  the  cells 
that  have  migrated  into  that  region,  by  the  second  and  third 
methods  of  reorganization  described  by  Roux.  When  the  mate- 
rial of  the  injured  half  is  only  incompletely  reorganized,  there 
is  formed,  after  postgeneration,  a  more  or  less  pronounced  spina 
bifida.  When  the  injured  material  is  completely  worked  over 
or  reorganized  and  postgenerated,  a  perfect  embryo  may  be 
formed. 

Schultze  ('94,  b,  d)  has  made  an  interesting  modification  of 
one  of  the  experiments  of  Pfliiger  and  obtained  most  unex- 
pected results.  The  eggs  of  Rana  fusca  removed  from  the 
uterus  were  placed  singly  upon  slides.  On  each  slide  had 
been  stuck  two  thin  glass  rods  from  1.65  to  1.35  mm.  in  thick- 
ness. Between  these  rods,  which  are  separated  from  each  other 
by  the  width  of  the  slide,  an  egg  is  placed  with  the  white  pole 
uppermost.  The  egg  is  then  fertilized  in  this  position.  After 
three  minutes  the  spermatozoa  may  be  supposed  to  have  entered, 
and  a  glass  cover  is  placed  over  the  egg  and  brought  down  into 
contact  with  the  two  glass  rods  above-mentioned,  and  there 
fixed  with  rubber  rings.  The  Qgg  is  by  this  means  slightly 
compressed  and  held  more  or  less  firmly  in  position.  Each 
slide  is  then  turned  over,  i.e.  through  180  degrees,  so  that  the 
dark  pole  of  the  compressed  Qgg  is  brought  upward.  The 
eggs  now  in  the  normal  position  are  put  into  a  dish  of  water, 
to  remain  in  this  position  until  the  jirst  furrow  has  appeared  or 
even  until  it  has  passed  through  the  egg.     Then  the  slide  and 


Ch.  XI]         EFFECT  OF  IXJURING  A  BLASTOMERE 


117 


its  egg  are  again  rotated  through  180  degrees^  so  that  the  white 
pole  is  ouce  more  turned  uppermost.  Owing  to  the  compres- 
sion, the  eggs  retain  their  inverted  position. 

After  twenty-four  hours  at  17  degrees  C,  the  gastrulation 
begins.     The  rubber  bands  are  then  removed  from  the  slide, 


Fig.  36. — Doable  embryos.  A.  Section  through  segmentiuo:  egg.  (After  Wetzel.) 
B.  Double  embryos  united  ventrally.  C,  D.  Double  embryos  united  dorsally. 
(After  Schultze.)  E.  Cross-section  through  C.  (After  Wetzel.)  F.  Double 
embryos  united  laterally,  and  G,  cross-section  of  same.     (After  Wetzel.) 

the  cover-slip  carefully  cut  away  from  the  jelly  of  the  Qgg^  and 
the  slide  and  egg  returned  to  the  water. 

If  eggs  that  have  been  inverted  after  the  two-cell  stage  are 


118  DEVELOPMENT   OF   THE   FROG'S   EGG  [Cii.  XI 

watched  during  the  later  cleavage -period,  it  will  be  found 
that  the  upper  white  surface  disappears,  and  often  a  whitish 
band  is  found  in  the  position  of  the  first  furrow.  Continuous 
observation  also  shows  that  the  white  hemisphere  may  slowly 
sink  to  one  side.  At  thirty  hours  the  blastopore  has  appeared 
in  the  normal  eggs,  while  on  the  inverted  eggs  two  gastrula- 
invaginations  are  found.  From  each  half  of  the  Q^g  a  more  or 
less  complete  embryo  may  develop  (Fig.  36,  B,  C,  D).  The  two 
"  double  monsters "  are  united  to  each  other  in  various  ways, 
often  with  the  two  ventral  surfaces  united  in  one  common  yolk- 
mass,  as  shown  in  Fig.  36,  B.  Another  of  these  double  forms 
is  shown  in  Fig.  36,  C,  D,  and  a  cross-section  through  the  body 
in  P'ig.  36,  E. 

The  details  of  these  experiments  of  Schultze  have  not  yet 
been  published.  The  method  of  gastrulation  of  the  halves  is 
not  clearly  explained,  nor  does  Schultze  explain  the  changes 
that  take  place  in  the  interior  of  the  blastomere  after  the 
rotation.  The  results  show,  however,  in  the  clearest  way  that 
each  half  of  the  Qgg^  after  the  first  division,  has  the  power  to 
develop  all  the  organs  of  a  single  embryo. 

Wetzel  ('95)  has  more  recently  studied  the  gastrulation-pro- 
cess  in  some  of  these  embryos  and  has  given  a  fuller  descrip- 
tion than  Schultze  of  the  origin  of  the  archenteron.  A  cross- 
section  through  the  blastula-stage  of  one  of  these  eggs  is  shown 
in  Fig.  36,  A.  Two  distinct  segmentation-cavities  are  present 
in  the  upper  or  white  hemisphere  of  the  Qgg.  The  centre  of  the 
double  blastula  is  filled  with  large  yolk-cells.  The  sides  are 
formed  of  smaller  cells  richer  in  protoplasm  and  pigment.  The 
structure  of  this  double  blastula  shows  that,  in  all  probability, 
the  contents  of  each  of  the  first  two  blastomeres  have  rotated 
after  the  inversion  of  the  egg  so  that  the  more  protoplasmic 
portions  have  come  to  lie  at  the  outer  and  upper  sides  of  each 
blastomere ;  while  the  heavier  yolk  has  sunken  down  to  the 
lower  surface  along  the  cell-wall  that  separated  the  first  two 
blastomeres  from  each  other. 

At  a  later  stage  a  depression  appears  on  the  surface  of  the 
Qgg  in  the  region  of  the  plane  that  separated  the  first  two  blas- 
tomeres from  each  other,  i.e.  approximately  in  the  j)lane  of  the 


Ch.  XI]  EFFECT   OF   INJURING   A   BLASTOMERE  119 

first  cleavage.  This  depression  or  groove  on  the  surface  may 
divide  at  either  end  into  two  distinct  and  independent  grooves. 
Cross-sections  through  such  an  egg  show  that  the  groove  on 
the  surface  is  the  result  of  an  invagination  to  form  an  archen- 
teron  in  each  half.  This  means  that  each  half-blastula  has 
begun  to  invaginate  along  the  common  line  of  contact  of  the 
halves.  Since  the  halves  are  in  contact,  the  overgrowth  of 
each  blastopore  is  impossible.  The  lips  of  the  blastopore  of 
each  half,  therefore,  have  extended  around  the  equator  of  the 
egg  as  in  the  spina-bifida  embryos.  A  medullary  fold  appears 
later  along  each  blastoporic  rim,  and  then  it  becomes  apparent 
that  two  embryos  are  present,  each  a  spina  bifida,  and  united 
by  a  common  central  yolk-mass  (Fig.  36,  C,  D,  E).  The  open 
dorsal  surfaces  of  these  two  embryos  are  turned  toward  each 
other  (Fig.  36,  E). 

This  seems  to  be  the  more  common  type  of  double  monster 
produced  from  these  eggs.  If,  however,  the  blastoporic  invagi- 
nations begin  at  different  regions  of  the  two  hemispheres,  many 
possible  variations  of  the  method  described  will  be  introduced ; 
Schultze  and  AVetzel  have  in  fact,  as  we  have  seen,  described 
several  forms  of  these  double  monsters.     (Fig.  36,  B,  F.) 

It  seemed  to  me  not  improbable  that  Schultze's  results 
explain  in  part  the  difference  in  the  results  of  the  experiments 
of  Roux  and  of  Hertwig.  If,  on  the  one  hand,  the  uninjured 
blastomere  retain  its  normal  position  after  the  operation, 
i.e.  with  the  black  pole  turned  upward,  then  there  should 
develop  a  half -embryo,  in  Roux's  sense.  On  the  other  hand, 
if,  after  the  operation,  the  position  of  the  egg  be  reversed  so 
that  the  white  pole  of  the  uninjured  blastomere  is  turned 
upward,  then  a  whole  embryo  of  half-size  might  develop.  In 
Roux's  experiment  it  is  probable  (although  not  explicitly 
stated)  that  the  black  hemisphere  always  remained  upward 
after  the  operation.  Hertwig  does  not  say  in  what  position 
the  eggs  lay  in  his  experiments.  He  only  says  that  in  the  blas- 
tula  and  gastrula  stage  the  heavier  injured  yolk  was  down,  and 
the  lighter  uninjured  blastomere  was  above.  If,  immediately 
after  the  operation,  the  eggs  lay  with  the  injured  blastomere 
below,  we  should  expect  some  change   to   take   place    in  the 


120  DEVELOPMENT   OF   THE   FROG'S   EGG  [Ch.  XI 

interior  of  the  uninjured  blastomere  as  a  result  of  its  oblique  or 
even  inverted  position;  hence  the  uninjured  blastomere  might 
develop  differently  than  it  would  have  done  had  it  retained  its 
normal  position  (as  in  Roux's  experiment).  In  this  way  we 
might  attempt  to  reconcile,  in  part,  the  different  results  of 
Roux  and  Hertwig.  I  cannot  but  think,  however,  that  the 
main  difference  is  due  to  the  partial  development  of  the  injured 
blastomere  in  many  of  Hertwig's  experiments,  so  that  cells  split 
off  from  the  injured  blastomere  took  part  in  the  formation  of 
the  embryo. 

In  1894  I  made  the  following  experiments  to  determine 
whether  one  of  the  first  two  blastomeres  could  give  rise 
to  a  half  or  to  a  whole  embryo,  according  to  the  condi- 
tions of  the  experiment.  One  of  the  first  two  blastomeres 
was  killed  with  a  hot  needle  in  the  way  described  by  Roux 
('93,  c).i 

In  some  of  the  eggs  the  black  pole  remained  upward  after 
the  operation;  other  eggs  were  rotated  after  the  operation, 
so  that  the  white  pole  was  turned  upward.  The  eggs  were 
closely  watched  for  several  hours,  in  order  to  ascertain  with 
certainty  whether  the  injured  half  divided  or  not.  In  those 
cases  in  which  this  happened,  the  eggs  in  question  were  elimi- 
nated from  the  experiment. 

The  eggs  were  placed  at  first  on  a  moistened  glass  plate  and 
kept  for  a  time  in  a  moist  atmosphere,  or  else  simply  thrown 
into  water.  The  results  seemed  to  be  the  same.  When  the 
black  pole  of  the  uninjured  blastomere  remained  up,  the  blas- 
tomere developed,  in  all  the  cases  observed,  into  a  half-emhryo. 
Conversely,  those  eggs  in  which  the  white  pole  was  turned 
upward,  formed,  in  most  cases,  whole  embryos  of  half -size.  In 
the  latter  case  the  cleavage  was  modified  in  consequence  of  the 
reversed  position  of  the  Qgg.  The  upturned  white  hemisphere 
produced  smaller  cells  than  the  lower  black  hemisphere,  point- 
ing unmistakably  to  a  rotation  of  the  fluid  contents  of  the 
blastomere. 

1  The  needle  was  heated  each  time  before  piercing  an  egg.  This  made  a 
greater  injury  to  the  blastomere  much  more  certain.  On  the  other  hand,  it 
lowered  the  percentage  of  embryos  obtained,  because  in  many  cases  the  other 
blastomere  was  probably  injured  also  by  the  heat. 


Ch.  XI]  EFFECT   OF   INJURING  A   BLASTOMERE  121 

The  half -embryos  and  the  whole  embryos  of  half-size  developed 
independently  of  the  yolk-mass  of  the  injured  side.  In  this 
respect  my  results  differed  very  materially  from  the  results  of 
Hertwig.  Many  of  Hertwig's  embryos  developed  in  connection 
Avith  the  injured  blastomere ;  mine,  on  the  contrary,  developed 
independently  of  the  injured  blastomere.  I  suspect,  as  I  have 
said,  that  this  difference  may  be  in  part  due  to  this,  that  Hert- 
wig did  not  carefully  remove  from  his  experiment  those  eggs 
in  which  the  injured  blastomere  continued  to  segment,  and  that 
cells  from  the  injured  blastomere  took  a  direct  part  in  the  sub- 
sequent development. 

In  one  of  my  experiments,  in  which  the  uninjured  blasto- 
mere had  been  reversed  after  the  operation,  it  developed  into  a 
half-embryo,  and  not  into  a  whole  embryo  of  half-size.  ^lore- 
over,  in  this  embryo  the  medullary  folds  appeared  on  the  white 
surface  of  the  egg^  showing  that  a  rotation  of  the  contents  of 
the  blastomere  must  have  taken  place.  We  must,  therefore, 
conclude  that  the  simple  fact  of  the  rotation  of  the  blastomere- 
contents  is  not,  in  itself,  the  determining  factor  as  to  whether 
a  Avhole  or  a  half -embryo  will  result,  but  probably  the  kind  of 
rotation  determines  this  result.  The  result  may  also  depend 
in  part,  I  think,  upon  how  far  the  contents  of  the  uninjured 
blastomere  have  retained,  after  the  operation,  their  organic 
connection  with  the  other  injured  blastomere. 

In  later  papers  Roux  stated  that  he  has  often  obtained  in  his 
experiment  other  sorts  of  embryos  than  those  he  first  described, 
which  he  calls  "hemiooholoplasten."  These  are  whole  embryos 
that  have  come  from  tlie  uninjured  blastomere  without  the 
postgeneration  of  the  other  injured  blastomere.  Roux  inter- 
prets these  as  embryos  "completely  postgenerated,"  with  only 
a  partial  use  of  material  from  the  other  side,  or  even  with  no 
material  from  the  injured  side.  Roux  affirms  that  he  has  seen 
all  intermediate  stages  between  those  embryos  that  have  used 
all  of  the  yolk-material  of  the  injured  side,  those  that  have  used 
only  a  part  of  the  material  of  the  injured  side,  and  those  that 
have  not  used  any  of  this  material.  These  embryos  differ  from 
one  another  only  in  point  of  size.  Roux  does  not  call  the  em- 
bryos that  have  developed  entirely  from  the  material  of  the 


122  DEVELOPMENT  OF  THE   FROG'S  EGG  [Ch.  XI 

non-injured  side,  whole  embryos  of  half -size,  but  he  believes 
that  at  first  there  formed  a  half-gastrula,  then  a  half-embryo. 
Later  this  half -embryo  completed  itself  without  using  material 
from  the  injured  side!  That  is  to  say,  by  using  "Avandering 
cells "  the  half-embryo  has  posfgeyierated  the  other  half  of  its 
body ! 


CHAPTER  XII 

INTERPRETATIONS    OF    THE    EXPERIMENTS;    AND   CON- 
CLUSIONS 

The  results  of  the  experiments  of  Pfliiger,  of  Roux,  and  of 
others  have  given  rise  to  much  discussion  in  respect  to  the 
relation  existing  between  the  unsegmented  egg  and  the  embryo. 
The  old  questions  of  evolution  and  epigenesis  have  been  once 
more  brought  into  the  foreground,  but  divested  of  their  historic 
meaning. 

The  results  of  the  experiments  on  the  frog's  Qgg  are,  how- 
ever, in  the  first  place,  too  insufficient  in  themselves,  and  in  the 
second  place  are  as  yet  too  uncertain  on  many  points,  to  warrant 
general  conclusions  based  on  these  results  alone.  The  experi- 
ments can  only  be  understood  if  considered  in  connection  with 
similar  experiments  on  other  groups  of  animals. 

Roux's  Mosaic  Theory  of  Development 

Roux's  discussion  of  the  problems  of  development  is  deserv- 
ing of  most  careful  examination,  for  even  in  his  earliest  papers 
we  see  foreshadowed  many  of  the  possible  interpretations  that 
have  later  been  accepted  in  one  or  another  form.  Roux  pointed 
out  that  the  known  facts  of  development  showed  that  a  certain 
formal  self-differentiation  of  many  parts  of  the  Qgg  takes  place 
during  development.  This  self-differentiation  may  result  from 
an  unequal  growth  of  different  substances  in  the  Qgg  which 
come  into  activity  at  different  times  ;  and  if  so,  it  should  be  our 
aim  to  discover  the  stimuli  that  bring  these  different  substances 
into  action,  and  thus  cause  the  consecutive  series  of  events. 
The  stimuli  must  come  either  from  without  at  each  stage  of 
development,  or  the  egg  may  contain  within  itself  the  power  of 
progressive  development  as  soon  as  it  is  once  set  into  activity. 
That  the  Qgg  needs  during  its  development  certain  things  from 

123 


124  DEVELOPMENT  OF   THE   FROG'S  EGG         [Ch.  XII 

its  environment  is  self-evident;  a  certain  amount  of  warmth 
and  of  oxygen,  etc.,  must  be  present.  These,  while  necessary 
for  the  development  of  the  egg^  do  not  necessarily  determine 
the  sequence  of  events  ;  for  under  the  same  external  conditions, 
eggs  of  different  animals  develop  very  differently.  The  results 
obtained  by  placing  the  frog's  egg  under  different  conditions 
also  show  that  the  power  of  progressive  development  must  lie 
within  the  egg  itself.  Roux  compared  the  egg,  in  this  respect, 
to  a  complicated  piece  of  machinery  which,  when  once  set  in 
motion,  would  go  through  a  long  series  of  changes  depending 
on  its  internal  structure. 

If  so  much  be  granted,  the  next  question  to  be  answered  is 
this  :  do  all  the  parts  of  the  dividing  egg  work  together,  i.e.  in- 
teract to  form  the  result,  or  have  the  parts  of  the  egg  separated 
from  one  another  by  the  cleavage  the  power  to  develop  inde- 
pendently? The  first  alternative  Roux  called  the  differentiat- 
ing interaction  of  the  parts,  and  the  latter  alternative,  the  self- 
differentiation  of  the  parts.  With  reference  to  the  results  of 
the  experiment  in  which  one  of  the  first  two  blastomeres  of  the 
frog's  egg  was  killed  or  injured,  Roux  concluded  that  each  of 
the  first  two  blastomeres  shows  in  this  experiment  the  power 
of  self-development :  i.e.  each  half  is  independent  of  the  other 
and  we  may  legitimately  infer  that  when  both  blastomeres  are 
alive,  as  in  the  normal  development,  the  same  self-differentia- 
tion of  each  blastomere  takes  place.  This  independent  devel-. 
opment  goes  on  till  the  organs  of  the  body  begin  to  form. 
Whether  the  limit  of  independent  development  is  then  reached 
we  do  not  know,  for  it  is  possible  that  in  the  complicated  series 
of  movements  tl\at  take  place  in  the  formation  of  some  of  the 
organs,  the  power  of  independent  development  may  be  sup- 
plemented or  replaced  by  the  action  resulting  from  the  cor- 
relation of  the  parts  to  one  another,  i.e.  by  a  mechanical 
interaction  of  different  parts.  Each  of  the  first  two  blasto- 
meres contains  not  only  the  building-material  for  the  corre- 
sponding parts  of  the  embryo,  but  also  the  differentiating  and 
formative  forces  for  those  parts.  The  cleavage  in  the  direct, 
or  normal  development  of  the  individual,  divides  qualitatively 
the  "  germ-plasm,"  and,  in  particular,  the  nuclear  material. 
The   development   of  the  frog's   gastrula  and  of   the    embryo 


Cu.  XII]        IXTERPRETATIOXS   AND   COXCLUSIOXS  125 

immediately  resulting  from  the  gastrula  is,  from  the  second 
furrow  on,  a  mosaic  work  of  at  least  four  vertical,  independent 
pieces.  How  far  this  mosaic  work  of  four  pieces  is  altered  by 
later  changes  in  the  position  of  the  material,  and  by  differentiat- 
ing correlation,  is  not  known. 

Roux  also  stated  clearly  the  relation  that  exists  between  the 
method  of  self-differentiation,  and  the  method  of  interaction 
of  the  parts  on  one  another,  and  the  bearing  of  these  questions 
on  the  older  problems  of  evolution  and  epigenesis.  If  many 
portions  of  the  egg  are  differentiated  owing  to  their  inherent 
power,  and  produce  in  this  way  the  manifold  differentiations 
seen  in  the  embryo,  then  the  egg  must  have  been  composed 
in  the  beginning  of  many  parts  bound  up  together,  and  the 
development  is  a  metamorphosis  or  an  unfolding  of  its  pecu- 
liarities ;  i.e.  the  development  is  an  evolution.  Further,  the 
cleavage  not  only  divides  the  egg  into  smaller  parts,  but  at 
the  same  time  localizes  the  differentiated  material,  so  that  this 
material  is  arranged  definitely  in  relation  to  later  development. 
This  result  appears  possible  only  through  a  qualitative  sepa- 
ration of  the  material  during  the  course  of  the  cleavage.  If 
this  is  true,  we  see  that  the  development  depends  on  the 
molecular  structure  of  the  egg^  and  therefore  further  analysis 
is  beyond  our  reach.  The  segmented  egg  would  be  then  only 
the  Sinn  of  its  independent  parts^  and  during  the  period  of  the 
self-differentiation  of  these  parts,  there  has  been  no  united 
action  to  form  a  whole.  Therefore  the  whole  can  have  no 
regulating  or  formative  influence  on  the  parts. 

If  this  view  be  true,  His's  principle  of  germinal  localization 
in  the  egg  has  not  only  a  descriptive  worth,  but  also  expresses 
a  causal  relation,  so  that  organs  can  be  referred  to  parts  of 
the  fertilized  egg^  and  even  to  the  unfertilized  egg.^ 

If,  on  the  other  hand,  development  takes  place  as  a  result 
of  the  interaction  of  all  or  many  parts  on  one  another,  then 
the  fertilized  egg  may  be  composed  of  a  very  few  differentiated 
parts,  which  by  their  interaction  produce  a  greater  and  greater 


1  We  could  explain  those  exceptional  cases  in  which  two  embryos  arise  from 
one  Qgg^  if  we  supposed  that  after  the  first  cleavage  there  was  a  sort  of  doubling, 
in  each  blastomere,  of  the  primary  constituents  of  the  body  (Roux). 


126  DEVELOPMENT   OF   THE   FROG'S   EGG  [Ch.  XII 

complexity.  The  development  would  then  be  due  to  the  pro- 
duction of  many  parts  out  of  a  few  primary  ones,  i.e.  the  de- 
velopment is  a  process  of  epigenesis.  There  would  thus  result 
an  ever-changing  interaction  of  the  parts  to  form  the  whole, 
by  which  means  there  would  be  also  brought  into  play  a  regu- 
lating influence  of  the  whole  back  again  on  the  parts,  i.e.  corre- 
lation of  the  parts  under  the  influence  of  the  whole.  His's 
principle  of  germinal  localization  would,  therefore,  have  a 
causal  meaning  only  in  so  far  as  it  points  out  the  place  in  the 
egg  where  the  resulting  formation  of  many-sided  changes  takes 
place ;  and  it  would  be  of  only  secondary  value  to  be  able  to 
refer  the  place  of  action  of  these  changes  to  the  undifl'eren- 
tiated  plasm  or  to  the  unfertilized  egg.^ 

In  conclusion,  it  should  be  noted,  Roux  said,  that  self -differ- 
entiation of  the  parts  and  dependent  differentiation  of  the  parts, 
i.e.  evolution  and  epigenesis,  may  be  combined  in  a  man3^-sided 
activity  or  union.,  and  it  would  then  be  our  duty,  in  order  to 
interpret  these  problems,  to  use  a  double  foresight  and  a  double 
care,  to  make  out  the  part  played  by  each  of  these  factors  in  the 
development. 

Theory  of  Driesch  and  of  Hertwig  of  the  Equiva- 
lency OF  THE  Early  Blastomeres 

Studies  on  other  forms  show  that  great  care  must  be  taken 
in  interpreting  the  results  of  the  experiments  on  the  frog's  Qgg. 
In  1891  Driesch  made  a  series  of  most  important  experiments 
on  the  eggs  of  the  sea-urchin. ^  The  blastomeres  were  isolated 
by  shaking  them  apart,  and  it  was  found  that  although  each 
blastomere  segmented  as  a  part,  i.e.  as  if  still  in  contact  with 
the  missing  half,  yet  the  open  side  of  the  blastula  closed  over 
very  soon,  and  a  gastrula  and  embryo  having  the  normal  form 
were  produced.  Driesch  concluded  from  this  and  similar  experi- 
ments that  all  the  blastomeres  are  equivalent,  and  that  the  posi- 
tion of  each  blastomere  in  the  segmenting  egg  determines  in 

1  The  formation  of  two  embryos  from  one  egg  would  take  place,  on  the  theory 
of  interaction  of  the  parts,  at  the  time  when  the  median  axis  of  the  body  is  formed. 
Two  such  axes  would  be  laid  down  instead  of  one. 

2  Fiedler  had  made,  in  1891,  a  somewhat  similar  experiment,  but  it  was  not 
earned  sufficiently  far  to  be  of  great  value. 


Ch.  XII]        IXTERPRETATIOXS  AXD  COXCLUSIOXS  127 

ereneral  the  fate  of  the  blastomere.  If  the  blastomeres  could  be 
interchanged  as  can  the  individual  marbles  of  a  heap,  then  the 
fate  of  each  would  be  determined  by  its  new  position  in  the 
whole.  This  conclusion  is  directly  opposed  to  Roux's  theory 
of  a  qualitative  division  of  the  blastomeres  during  the  early 
cleavage. 

Hertwig  had  also  stated  shortly  before  Driesch  that  in  his 
opinion  the  egg  divides  quantitatively,  and  that  Roux's  experi- 
ments did  not  touch  the  cardinal  point  of  this  problem,  because 
the  other  injured  half  of  the  egg  remained  in  contact  with  the 
developing  half.  Hertwig  expressed  his  belief  that  if  the  first 
two  blastomeres  of  the  frog's  egg  could  be  separated  from  each 
other,  each  would  develop  into  a  whole  embryo.  Further,  he 
thought  that  the  development  of  an  organism  is  not  a  mosaic 
work,  but  that  the  parts  develop  in  relation  to  one  another,  i.e. 
the  development  of  a  part  is  dependent  on  the  development 
of  the  whole.  Wilson  ('93)  also,  from  the  results  of  a  most 
careful  and  important  series  of  experiments  on  the  egg  of 
Amphioxus,  concluded  that  the  division  of  the  egg  is  not  quali- 
tative. He  found  that  isolated  blastomeres  give  rise  to  larvte 
smaller  in  size  than  the  normal,  but  having  the  normal  form. 
The  differentiation  of  the  blastomeres,  Wilson  thought,  takes 
place  in  the  later  periods  of  cleavage. 

Roux's  Subsidiary  Hypothesis 

Roux  replied  to  the  criticisms  that  Driesch,  Hertwig,  Wil- 
son, and  others  have  made  of  his  theory,  and  attempted  to  show 
that  his  view  is  fully  compatible  with  the  results  that  Driesch 
and  others  obtained. 

Roux  ('92,  a,  '98,  b)  pointed  out  that  the  results  of  Chabr},^ 
Fiedler,  and  Chun  show  that  in  ascidians,  sea-urchins,  and 
ctenophors  a  half-development  takes  place  when  one  of  the 
first  two  blastomeres  has  been  removed,  and  that  the  experi- 
ments of  Driesch  also  showed  that  an  isolated  blastomere 
of  the  egg  of  Echinus  cleaved  as  a  half,  and  not  as  a  whole, 
and  that  a  half-blastula  also  developed.      These  results  indi- 

1  Later  experiments  have  shown  that  this  statement  is  not  true  for  ascidians, 
as  Chabry's  work  might  seem,  in  part,  to  show. 


128  DEVELOPMENT  OF   THE  FROG'S  EGG  [Ch.  XII 

cate  that  a  certain  formal  self-differentiation  of  many  parts 
of  the  segmenting  egg  has  taken  place.  On  the  other  hand, 
the  fact  of  postgeneration  shows  that  in  each  of  the  hrst 
blastomeres  a  power  sufficient  to  complete  the  whole  must 
also  be  potentially  present.  In  order  to  awaken  this  potential 
power  of  a  blastomere,  a  disturbance  in  the  development  must 
occur.  This  latent  activity  may  be  only  slowly  awakened  in 
the  development,  sometimes  sooner,  sometimes  later. 

We  have,  therefore,  to  distinguish  two  sorts  of  development, 
—  the  normal  "direct"  development,  and  an  "indirect"  post- 
generative  (or  regenerative)  development.  The  first  or  direct 
is  the  result  of  the  self -differentiation  of  the  early  blastomeres, 
and  of  the  complexity  of  their  derivatives.  The  second  or 
indirect  is  the  result  of  a  profound  correlation  which  adds  to 
an  imperfect  whole  the  lacking  parts.  Should  the  postgenera- 
tion set  in  immediately  after  the  isolation  of  the  blastomere 
and  so  convert  the  blastomere  at  once  into  an  actual  whole, 
then  we  should  not  have  found  out  that  each  blastomere  is 
really  a  self-differentiating  cell,  but  we  should  have  erroneously 
concluded  that  the  first  (four)  cleavage-cells  are  qualitatively 
equivalent.  Into  this  error  Roux  believed  Driesch  and  Hert- 
wig  to  have  fallen.  In  the  frog,  ascidian,  and  ctenophor  each 
of  the  first  blastomeres  is  specifically  different  from  the  others, 
but  in  respect  to  postgeneration  we  find  that  each  blastomere 
has  the  same  potentiality,  and  each  is  in  reality  totipotent. 
The  "idioplasm"  in  direct  (i.e.  normal)  development,  called 
into  activity  by  the  process  of  fertilization,  is  divided  qualita- 
tively and  unequally  during  the  cleavage,  while  the  material 
which  may  later  serve  for  postgeneration  and  regeneration 
(which  is  not  active  during  the  normal  development)  is  always 
equally  or  quantitatively  divided. 

According  to  Roux,  the  nucleus  represents  the  controlling 
power  of  the  cell,  but  the  protoplasm  acts  as  a  stimulus  to 
the  nucleus  and  hence  may  indirectly  regulate  the  process  of 
cleavage.  "In  the  telolecithal  frog's  Qgg  the  position  of  the 
food-substances  and  formative  substances  stands  in  strict  causal 
relation  to  the  position  of  the  main  axes  of  the  embryo."  The 
nuclei  of  eggs  in  wdiich  the  normal  arrangement  of  the  contents 
has  been  disturbed  will  be  influenced  during  the  first  cleavage- 


Ch.  XII]        INTERPRETATIOXS  AND  CONCLUSIONS  129 

period,  so  that  a  qualitative  division  of  the  nucleus  may  result 
different  from  the  corresponding  normal  qualitative  division. 
The  second  cleavage,  for  instance,  may  come  first  (qualitatively) 
as  a  result  of  the  position  of  the  nucleus  in  the  protoplasm. 

Roux  further  suggested  that  the  consecutive  series  of  nuclear 
divisions  must  be  different  m  kind  in  the  normal  and  in  the 
compressed  eggs,  and  that  an  "  anachronism  "  has  taken  place 
in  the  latter  case.  By  this  "  anachronism ''  Roux  has  tried  to 
save  his  theory  of  qualitative  division  of  the  nucleus  during 
the  cleavage-period. 

To  sum  up  Roux's  later  position,  we  may  say  that  in  order 
to  vindicate  his  earlier  theory  of  a  qualitative  division  of  the 
nucleus  and  a  resulting  self-differentiation  of  the  first-formed 
blastomeres,  he  has  been  obliged  in  the  first  place  to  bring  for- 
ward his  theory  of  postgeneration,  assuming  that  along  with  the 
qualitative  division  of  the  nucleus  a  parallel  quantitative  divi- 
sion of  the  germ-material  also  occurs.  Further,  Roux  assumes 
that  the  kind  of  qualitative  division  of  the  nucleus  is  directly 
influenced  by  the  arrangement  of  the  protoplasm,  and,  as  we 
have  seen  above,  he  is  unable  to  explain  satisfactorily  the 
results  of  the  experiment  of  the  compressed  Qgg^  except  as  an 
"anachronism."  These  complications  into  which  Roux  has 
been  forced  are  largely  the  outcome  of  the  primary  assump- 
tion of  a  qualitative  division  of  the  nucleus.  This  Roux-Weis- 
mann  hypothesis  of  qualitative  nuclear  division  has,  however, 
no  known  histological  facts  in  its  favor.  On  the  contrary,  all 
we  know  of  nuclear  divisions  speaks  clearly  in  favor  of  an  exact 
division  of  the  chromatin-material,  and  a  most  elaborate  mech- 
anism is  present  to  bring  about  this  result. 

Experiments  on  Other  For:ms 

The  results  obtained  from  a  study  of  the  development  of 
fragments  of  the  unsegmented  Qgg  and  of  isolated  blastomeres 
of  ctenophors^  have  a  direct  bearing  on  our  interpretation  of  the 
experiments  on  the  frog's  Qgg.  When  the  first  two  blastomeres 
are  separated  from  each  other  by  a  sharp  needle  or  cut  apart  by 
a  pair  of  small  scissors,  each  continues  to  cleave  as  a  half,  i.e, 

1  Chun  ('92).     Driesch  and  Morgan  ('95). 


130  DEVELOPMENT   OF   THE   FROG'S  EGG  [Ch.  XII 

as  though  it  were  still  in  contact  Avith  its  fellow-blastomere. 
When  the  organs  apjDcar  in  the  larva,  only  half  the  full  num- 
ber of  rows  of  swimming-paddles  appear.  Each  row,  however, 
has  its  full  complement  of  paddles.  The  invagination  of  ecto- 
derm to  form  the  '' stomach"  is  very  excentric  in  the  half -larva, 
but  forms  a  closed  tube  running  from  the  mouth-opening  to  the 
excentric  sense-plate.  In  several  respects,  therefore,  the  larvae 
were  distinctly  half-larvte.  But  in  another  respect  they  were 
more  than  half -larvae.  The  endodermal  cells  of  the  normal 
larva  arrange  themselves  into  four  hollow  pouches,  and  the 
"  stomach  "  invagination  passes  in  the  central  line  of  the  four 
pouches.  In  the  half -larva,  on  the  contrary,  the  endodermal 
mass  forms  more  than  two  pouches  {i.e.  more  than  half  the 
normal  number  in  the  whole  larva).  Two  distinct  pouches 
are  present  and  in  addition,  generally,  a  third  smaller  pouch  is 
formed.  The  latter  lies  excentrically.  In  the  meeting-point 
of  the  three  pouches  is  the  excentric  "  stomach  "  invagination. 

The  isolated  one-fourth  blastomere  segments  also  as  a  part 
of  a  whole,  and  develops  in  some  cases  into  a  one-fourth  larva, 
having  only  two  rows  of  paddles  (^i.e.  one-fourth  the  normal  num- 
ber), but  with  tivo  endodermal  pouches  (i.e.  with  one  more  than 
one-fourth  the  normal  number).  The  three-fourth  embryos 
develop  six  rows  of  paddles  (i.e.  three-fourths  of  the  normal 
number)  and  four  endodermal  pouches.  The  problem  is  here 
a  complicated  one,  for  while  in  one  set  of  organs  we  find  a  half- 
development,  in  other  organs  we  find  more  than  a  half,  but  yet 
not  the  whole  development. 

The  results  show,  however,  beyond  question,  that,  even  when 
isolated  from  its  fellow,  the  one-half  blastomere  may  give  rise 
to  a  larva  that  is  in  many  respects  only  one-half  of  the  normal 
larva. 

There  is  yet  to  be  described  another  series  of  experiments 
that  have  a  direct  bearing  on  the  interpretation  of  the  preced- 
ing results.  Roux  showed  that  if  a  part  of  the  protoplasm  be 
removed  from  the  unsegmented  frog's  egg.,  the  egg  may  continue 
in  many  cases  to  develop  into  a  normal  embryo.  The  eggs  of 
the  sea-urchin  lend  themselves  much  more  readily  to  this  ex- 
periment.    They  may  be  broken  up  into  fragments  of  all  sizes 


Ch.  XII]        INTERPRETATIONS   AND  CONCLUSIONS  131 

if  shaken  in  a  small  tube.  ^  Those  fragments  which  contain  the 
egg-nucleus  may  be  fertilized  and  will  develop.  If  the  pieces 
are  large  enough  a  gastrula  is  formed,  and  still  larger  pieces 
develop  into  normally  formed  larvae. 

When  the  unsegmented  egg  of  the  ctenophor  is  cut  into  pieces, 
there  may  result  either  a  whole  larva  or  a  larva  lacking  certain 
parts,  and,  further,  the  study  of  the  cleavage  of  these  egg- 
fragments  shows  that  if  the  fragment  cleaves  like  the  whole 
Qgg  (but  with  smaller  blastomeres)  then  a  whole  larva  results, 
while  if  the  cleavage  is  irregular  the  larva  is  also  imperfect. 
Presumably,  in  the  first  case  the  egg  has  been  cut  symmetri- 
cally, but  in  the  second  case  unsymmetrically.  Or  we  might 
assume  that  in  the  one  case  the  egg-fragment  rearranged  its 
protoplasm  into  a  new  whole,  while  in  the  second  case  it 
was  unable  to  do  so.  On  either  alternative  we  must  conclude 
that  a  defect  in  the  protoplasm  often  brings  about  a  modified 
cleavage  and  also  a  defective  embryo,  and  this  takes  place  even 
although  the  ivhole  of  the  nuclear  material  of  the  unsegmented  egg 
remains  present.  There  seems,  therefore,  no  escape  from  the 
conclusion  that  in  the  protoplasm  and  not  in  the  ?iucleus  lies  the 
differentiating  poicer  of  the  early  stages  of  development. 

General  Conclusions 

We  have  seen  that  one  of  the  first  two  blastomeres  of  the 
frog's  Qgg  may  develop  into  a  half-embryo,  or  into  a  whole 
embryo  of  half-size,  according  to  the  conditions  of  the  experi- 
ment. So  long  as  the  first  two  blastomeres  remain  in  contact 
without  any  disturbance  of  the  cell-contents,  each  blastomere 
develops  its  half  of  the  body.  On  the  other  hand,  if  the  proto- 
plasm is  disturbed  by  reversing  the  position  of  the  Qgg  after 
the  first  cleavage,  there  generally  results  a  whole  embryo  from 
each  blastomere.  Unfortunately,  it  has  not  been  found  possible 
to  separate  completely  from  each  other  the  first  two  blasto- 
meres of  the  frog's  Qgg^  so  that  we  do  not  know  whether 
a  whole  embryo  of  half-size  or  a  half-embryo  would  result. 
In  other  animals  (Echinodermata,  Hydromedusae,  Teleostei, 
Amphioxus,  Ascidia,  and  Salamandra)  each  of  the  first  two 
blastomeres,  if  separated  from  its  fellow,  develops  into  a  whole 
embryo,  regardless  of   the   means    employed   to   separate   the 


132  DEVELOPMENT   OF   THE   FROG'S   EGG  [Cii.  Xll 

blastomeres.^  On  the  other  hand,  tlie  isolated  hlastomere  of  the 
ctenophor-egg  develops  into  a  half-embryo.  These  experiments 
show  that  the  half-development  of  the  frog's  Q<^g  need  not  be 
the  result  of  the  presence  of  the  other  blastomere,  as  has  been 
suggested.  This  is  also  shown  by  Schultze's  experiment,  in 
Avhich,  although  both  halves  are  present  and  in  contact,  each 
blastomere  develops  into  a  whole  embryo. 

The  results  show  that  in  general  the  first  blastomeres  are 
totipotent,  i.e.  each  has  the  power  to  produce  a  whole  embryo 
if  separated  from  its  fellow,  even  although  it  may  under  cer- 
tain conditions  produce  only  a  half-embryo,  as  in  the  frog. 
Nevertheless,  in  most  forms  each  isolated  blastomere  continues 
to  segment  as  though  still  in  contact  with  the  other  half.  This 
latter  phenomenon  shows  that  the  egg-protoplasm  has  a  defi- 
nite arrangement  according  to  which  the  cleavage  peculiar 
to  each  kind  of  Qgg  is  brought  about,  and  there  is  sufficient  evi- 
dence to  show,  I  think,  that  this  is  a  cytoplasmic  phenomenon, 
and  is  not  the  result  of  nuclear  interference.  We  have  also 
seen  that  some  of  the  isolated  blastomeres  that  cleave  as  a 
part  may  later  develop  into  whole  larvie  (echinoderms),  while 
other  blastomeres  that  cleave  as  a  part  may  give  rise  to  half- 
larvse  (ctenophors).  That  these  phenomena  too  are  dependent 
directly  on  the  cytoplasm  is  shown  by  the  experiment  of  cut- 
ting a  piece  from  the  unsegmented  Qgg.  Under  these  circum- 
stances, the  nucleated  fragment  of  the  echinoderm-egg  gives 
rise  to  a  whole  embryo,  although  it  segments  as  a  part,  while  in 
the  ctenophor  an  imperfect  embryo  is  generally  formed.  The 
results  in  these  two  cases  are  nearly  the  same  as  when  the  blas- 
tomeres of  the  respective  eggs  are  isolated,  although  in  the 
latter  experiment  the  entire  segmentation-nucleus  is  present. 
In  the    ctenophor  the   process   of  self-regulation  seems  to  be 


1  Roux  ('95)  has  stated  that  the  development  of  a  half  or  a  whole  embryo 
may  depend  upon  the  method  employed  to  separate  the  blastomeres.  If  shaken 
apart,  whole  embryos  result;  if  cut  apart,  half-embryos.  Zoja's  results  ('95) 
refute  such  an  interpretation.  He  cut  apart  echinoderm  and  hydroid  eggs  and 
yet  got  whole  embryos.  On  the  other  hand,  when  the  blastomeres  of  the  cteno- 
phor are  cut  apart,  half-embryos  result.  It  must,  however,  be  admitted  that 
disturbance  of  the  contents  of  an  isolated  blastomere  might  be  favorable  to 
whole  development,  as  in  the  frog. 


Cii.  XII]        INTERPRETATIONS   AND   CONCLUSIONS  I33 

largely  absent,  either  because  the  blastomeres  cannot  be  brought 
into  a  new  whole,  or  because  the  protoplasm  is  so  fixed,  so  stiff, 
that  it  cannot  readily  rearrange  itself.  If  from  either  of  these 
conditions,  or  from  some  other,  the  blastomeres  are  not  capable 
of  rearrangement  or  reconstruction,  an  imperfect  embr3*o  re- 
sults. 

How  far  does  the  totipotence  of  the  blastomeres  reach?  Does 
it  end  with  the  two-cell,  four-cell,  or  later  stacjes  of  cleavaGfe  ? 
Probably  this  varies  in  different  eggs.  The  one-fourth  blasto- 
mere  of  Echinus  can  form  a  perfect  embryo,  and  even  the  one- 
eighth  blastomere  may  develop  into  a  gastrula.  Tlie  same  is 
true  for  the  egg  of  Amphioxus.  For  the  frog  it  is  not  yet 
possible  to  say  where  the  limit  lies.  In  this  connection  the 
following  facts  are  of  importance.  The  isolated  blastomere 
of  the  sea-urchin's  egg  runs  through  the  same  number  of  divi- 
sions that  it  would  have  done  had  it  remained  in  contact  with 
its  fellows.  1  Hence  the  half-embryo  has  only  one-half  the 
number  of  cells  of  the  normal  embryo,  and  the  one-fourth  em- 
bryo has  only  about  one-fourth  the  full  number.  This  seems 
to  give,  in  part,  an  explanation  of  the  statement  made  above, 
viz.,  that  the  one-half  embryo  develops  further  than  the  one- 
fourth,  and  the  latter  further  than  the  one-eighth,  since  the 
smaller  the  isolated  blastomere  the  fewer  are  the  cells  it  pro- 
duces from  which  the  embrjo  is  formed.  The  lack  of  power  of 
development  of  the  small  isolated  blastomere  is  not,  therefore, 
dependent  on  its  differentiation.  This  is  also  shown  by  the  fol- 
lowing experiment.  In  the  blastula-stage  of  the  sea-urchin's 
egg,  pieces  may  be  cut  or  shaken  from  the  blastula-wall,  and, 
if  large  enough,  they  develop  into  small  larvge.  Here  also  we 
find  that  the  large  pieces  can  go  further  in  the  ontogeny  than 
the  smaller  pieces,  probably  owing  to  the  presence  of  a  sufficient 
number  of  cells  or  of  sufficient  material  to  form  the  necessary 
organs  of  the  embryo. ^ 

If  the  early  blastomeres  are  totipotent,  what  brings  about  the 
later  differentiation  of  these  cells  ?    There  are  sufficient  reasons. 


1  Morgan  ('95). 

2  The  same  experiment  cannot  be  made  on  the  frog's  blastula,  because,   if 
cut,  the  pieces  immediately  disintegrate. 


134  DEVELOPMENT   OF   THE   FROG'S   EGG         [Ch.  XII 

I  think,  to  conclude  that  the  power  of  differentiation  lies  within 
the  egg  itself,  and  does  not  depend  directly  on  external  stimuli. 
We  have  seen  that  Roux  and  Weismann  (particularly  the  latter) 
explain  this  differentiation  of  the  cells  as  a  result  of  the  quali- 
tative division  of  the  nucleus  from  the  very  beginning  of  the 
cleavage.  The  nucleus,  unravelling  its  qualities  at  each  divi- 
sion, sends  into  each  cell  the  proper  constituents,  and  the  nuclei, 
then  acting  on  the  cell-protoplasm,  cause  it  to  differentiate. 
On  the  other  hand,  Hertwig  contends  that  the  early  blastomeres 
are  equivalent,  and  that  differentiation  is  brought  about  by  the 
interaction  of  the  blastomeres.  In  other  words,  any  blastomere 
that  has  come  to  occupy  a  given  position  has  its  fate  sealed,  be- 
cause in  this  position  it  bears  a  certain  relation  to  the  other 
blastomeres  of  the  whole  ;  the  whole  being  simply  the  sum-total 
of  the  blastomeres  present.  But  it  is  impossible  to  imagine  that 
the  interaction  of  strictly  equivalent  blastomeres  could  bring 
about  a  self -differentiation.  If  it  is  assumed  that  the  gross- 
contents  (such  substances  as  yolk,  etc.)  determine  the  differ- 
entiation of  each  part,  still  the  hypothesis  is  obviously  insuffi- 
cient for  all  cases,  because,  as  we  have  seen,  fragments  of  a7it/ 
part  of  the  egg  of  echinoderms  develop  into  whole  embryos, 
and  fragments  even  of  the  blastula  form  new  blastulse,  gas- 
trulse,  and  embryos.  Some  of  these  small  blastulse  represent 
only  the  ''  animal "  half  of  the  original  blastula,  and  the  cells 
will  not,  therefore,  contain  any  of  the  protoplasm  or  yolk  that 
the  cells  usually  contain  that  are  invaginated,  for  all  this  por- 
tion of  the  blastula  has  been  cut  off.  And  since  these  "ani- 
mal" pieces  gastrulate,  we  must  infer  that  the  gross-contents  of 
the  blastomere,  or  collection  of  blastomeres,  do  not  necessarily/ 
cause  the  differentiation.  If,  then,  neither  qualitative  division 
of  the  nucleus,  nor  cellular  interaction,  nor  the  gross-contents 
of  the  blastomeres  can  be  the  cause  of  differentiation  of  the 
embryo,  what  does  bring  about  the  differentiation  ?  There  are 
certain  facts  of  inheritance  that  also  have  a  bearing  on  this  ques- 
tion. The  characters  of  the  male  are  known  to  be  transmitted 
by  means  of  the  spermatozoon.  The  latter  carries  into  the  egg 
mainly  the  male  nucleus.  Therefore,  many  embryologists  have 
turned  to  the  nucleus  as  the  originator  of  the  differentiation  of 
the  cell.     Various  suggestions  have  been  offered  as  to  the  way 


Ch.  XII]        IXTERPRETATIOXS   AXD   COXCLUSIOXS  135 

in  which  such  an  influence  could  be  transmitted  from  the  nu- 
cleus to  the  cytoplasm.  Strasburger  supposes  the  nucleus  ex- 
erts a  dynamic  influence  on  the  cell-plasm.  De  Vries  and  others 
imagine  that  organized  particles,  "pangens,"  pass  out  of  the 
nucleus  to  transform  the  cytoplasm.  Driesch  suggests  that  the 
nucleus  secretes  ferments  which  change  the  cell-plasm.  These 
hypotheses  are  purely  imaginary,  for  at  present  we  know  almost 
nothing  of  the  function  of  the  nucleus ;  and  even  if  we  suppose 
the  differentiation  comes  in  some  unknown  way  from  the  nucleus, 
still  we  do  not  know  what  could  start  the  process  in  isolated 
nuclei  that  are  after  the  cleavage-period  assumed  to  be  equiva- 
lent. There  is,  however,  one  series  of  experiments  which  seems 
to  throw  some  light  on  the  present  problem,  although  the  inter- 
pretation is  extremely  difficult  and  hazardous.  I  refer  to  the 
experiment  on  the  ctenophor-egg,  in  which  a  part  of  the  cyto- 
plasm was  cut  from  the  unsegmented  egg,  and  the  latter  gave 
rise  in  most  cases  to  an  imperfect  embryo.  Here,  although  the 
entire  segmentation-nucleus  is  present,  yet  by  loss  of  cytoplasm 
defects  are  produced  in  the  embryo.  The  form,  therefore,  of 
the  early  embryo  w^ould  seem  to  result  from  the  structure  of 
the  protoplasm,  or  from  the  arrangement  of  the  blastomeres 
after  cleavage.  In  either  case  the  phenomenon  is  in  the  first 
instance  cytoplasmic.  How  can  this  conclusion  be  brought  into 
harmony  with  the  facts,  stated  above,  of  inheritance  of  charac- 
ters through  the  male  pronucleus  ?  Let  us  assume  an  imaginary 
case  to  show  how  this  union  of  the  two  conceptions  is  possible. 
If  we  had  used  the  spermatozoon  of  one  species  (or  variety)  of 
ctenophor  and  the  egg  of  another  species,  and  then  after  fertili- 
zation had  removed  a  part  of  the  egg-cytoplasm,  we  should  ex- 
pect to  find  the  embryo  defective,  but  the  organs  that  were 
formed  w^e  should  expect  to  show  a  combination  of  male  and 
female  characters.  In  other  words,  the  imperfect  embryo  w^ould 
have  resulted  from  the  arrangement  of  the  protoplasm  into  an 
imperfect  form,  but  the  kind  of  organ  would  have  depended  on 
the  structure  of  the  nucleus  in  each  cell.  After  cleavage,  the 
cytoplasm  of  each  part  differentiates  into  this  or  that  organ, 
but  the  kind  of  differentiation  of  each  part  is  determined  by  the 
nucleus  of  that  part. 

If  the  argument  given  above  should  prove  true,  then   the 


136  DEVELOPMENT   OF   THE   FROG'S   EGG         [Ch.  XII 

origin  of  the  differentiation  is  to  be  found  in  the  ultimate  struct- 
ure of  tlie  cytopLasm  of  the  egg  or  embryo,  although  even  then 
we  do  not  know  how  this  mechanism  could  be  started.  Whit- 
man ('95)  has  stated  his  conviction  that  it  is  erroneous  to  think 
of  the  embryo  as  only  the  sum-total  of  cells  interacting  upon 
one  another,  but  that  the  embryo  itself  is  to  be  thought  of  as  a 
whole,  which  regulates  its  parts  regardless  of  cell-boundaries. 
According  to  this  view,  each  portion  of  the  embryo  has  its  fate 
sealed,  not  because  the  given  portion  forms  a  member  of  the 
community  of  cells,  but  because  the  whole  directs  the  fate  of 
each  special  part.  Driesch  has  pointed  out  that  the  egg  seems 
to  act  like  an  intelligent  being.  If  so,  are  the  causes  of  dif- 
ferentiation and  of  regeneration  the  same  in  kind  as  physico- 
chemical  causes,  or  do  they  belong  to  the  category  of  intelligent 
acts,  and  can  these  latter  be  accounted  for  by  the  known  princi- 
ples of  chemistry  and  physics  ?  The  plain  answer  is,  we  do  not 
know. 


CHAPTER   XIII 

ORGANS  FROM   THE  ENDODERM 

We  may  now  turn  again  to  the  history  of  the  development 
of  the  normal  embryo. 

The  Closure  of  the  Blastopore,  and  the  Formation 

OF    THE   NeURENTERIC   CaNAL 

During  the  last  stages  of  the  closure  of  the  blastopore  its 
lateral  lips  rapidly  approach  each  other,  and  it  then  becomes 
an  elliptical  and  later  a  slit-like  opening  (Fig.  23).  The  pos- 
terior edge  of  the  blastopore  also  grows  forward  for  a  short 
distance,  and  as  a  result  a  pocket-like  continuation  of  the 
archenteron  is  formed  (Fig.  37,  A).  The  depth  of  this  pocket 
corresponds  to  the  extent  of  the  forward  growth  of  the  poste- 
rior edge  or  ventral  lip  of  the  blastopore.  If  the  embryo  be 
examined  in  the  region  over  which  the  posterior  lip  of  the  blasto- 
pore has  advanced,  there  will  be  found  at  first  nothing  on  the 
surface  to  mark  the  region  closed  over.  Some  observers  have 
described  faint  traces  of  a  groove  in  this  region,  but  such 
appearances  are  probably  exceptional.  Later,  however,  when 
the  outlines  of  the  medullary  folds  have  appeared,  a  distinct 
longitudinal  groove  appears  in  this  region  running  posteriorly 
from  the  small  blastopore  (Fig.  23,  B).  At  the  ventral  end 
of  the  groove  a  distinct  depression  or  pit  is  soon  formed 
(Fig.  37),  Avhich  marks  the  beginning  of  the  anus.  It  lies 
at  a  point  opposite  to  the  bottom  of  the  posterior  pocket  of 
the  archenteron,  and  corresponds  therefore  approximately  to 
the  region  at  which  the  first  trace  of  the  ventral  lip  of  the 
blastopore  was  found. 

As  the  medullary  folds  close  in  to  form  the  nervous  system, 
the  blastopore  is  overarched  by  their  posterior  ends.    The  folds 

137 


138 


DEVELOPMENT   OF   THE   FROG'S   EGG         [Cir.  XIII 


meet  above  and  posterior  to  the  blastopore,  so  that  the  hatter 
can  no  longer  be  seen  from  the  surface  (Figs.  23,  D,  E,  and 
37,  A).     As  a  result  the  central  canal  of  the  nervous  system 


He 


LV  PH 


NT 


Fig.  37.  —  Sagittal  sections  through  two  stages :  A.  when  blastopore  is  overarched  ; 
B.  when  anus  has  formed.  (After  Marshall,  with  modifications  in  A.)  A.  Anus. 
Fb.  Fore-brain.  Hb.  Hind-brain.  LV.  Liver-diverticulum.  Mb.  Mid-brain. 
N.  Notochord.  NT.  Neurenteric  canal.  PD.  Proctodaeum.  PH.  Pharynx. 
PN.   Pineal  body.      PT.  Pituitary  body. 

becomes  continuous  at  its  posterior  end  with  the  overarched 
blastopore,  and  by  means  of  the  latter  the  so-called  neurenteric 


Ch.  XIII]  ORGAXS   FROM   THE   ENDODERM  I39 

canal,  the  central  canal  of  the  nerve  tube,  is  directly  continued 
into  the  archenteron  (Fig.  37,  A).  At  this  time  the  archen- 
teron  is  completely  closed  in  from  the  exterior,  since  neither 
the  mouth  nor  the  anus  has  as  yet  opened. 

The  posterior  ends  of  the  medullary  folds  close  just  behind 
the  blastopore.  The  groove  lying  behind  the  blastopore  is  not 
overarched  by  the  folds.  During  this  period  the  posterior  pit 
of  this  groove  has  become  much  deeper.  At  first,  the  pit  was 
separated  from  the  archenteron  by  a  thick  layer  of  cells  con- 
sisting of  ectoderm,  mesoderm,  and  endoderm.  The  meso- 
dermal cells  begin  to  pull  away  from  this  region,  and  the  pit, 
in  consequence,  becomes  deeper.  Then  the  endodermal  cells 
pull  away  beneath  the  pit,  and  only  a  single  layer  of  ecto- 
dermal cells  remains  to  separate  the  cavity  of  the  archenteron 
from  the  exterior.  Finally  the  latter  cells  also  draw  away, 
and  the  pit  opens  into  the  archenteron.  The  external  opening 
becomes  the  anus  of  the  frog.  It  is  at  first  almost  on  the 
dorsal  surface  of  the  embryo,  but  it  rapidly  shifts^  to  a  more 
ventral  position,  and  at  the  same  time  the  region  above  it 
elongates  to  form  the  beginning  of  the  tail.  The  neurenteric 
canal  is  only  a  temporary  structure,  and  is  soon  obliterated  by 
the  growing  together  of  its  walls,  although  its  position  may 
be  marked  in  sections  for  some  time  after  its  actual  closure  by 
the  irregular  line  of  pigment  in  the  region  of  the  coalescence 
of  its  walls. 

In  the  Urodela  the  changes  that  take  place  during  the 
final  stages  of  the  blastopore  are  somewhat  simpler.  The 
circular  blastopore  is  reduced  to  an  elongated  slit-like  open- 
ing ;  but  there  seems  to  be  some  variation  in  the  details  of  the 
method  of  its  later  reduction.  The  medullary  folds  arch  over 
only  the  anterior  end  of  the  elongated  blastopore,  leaving  free 
the  posterior  end.  The  anterior  end  becomes  the  neurenteric 
canal.  The  sides  of  the  middle  part  of  the  slit-like  blastopore 
come  together  and  fuse  at  the  time  of  overgrowth  of  the  med- 
ullary folds.  The  posterior  end  of  the  blastopore  always 
remains  open  to  the  exterior,  and  forms  the  permanent  anus. 


1  The  method  by  which  the  apparent  change  in  position  of  the  anal  opening 
takes  place  has  not  been  clearly  made  out. 


140  DEVELOPMENT   OF   THE   FROG'S   EGG        [Ch.  XIII 

The  main  differences  that  exist  between  the  methods  of  forma- 
tion of  neurenteric  canal  and  anus  in  the  frog  and  in  urodeles 
are  these:  In  the  frog  the  ventral  lip  of  the  blastopore  grows 
forward  during  the  closure  of  the  blastopore,  and  only  subse- 
quently a  new  opening  forms  at  the  point  from  which  the  for- 


FiG.  38.  —  Embryo  of  Rana  tempoiaria  at  time  of  hatching. 

ward  growth  began  (Fig.  37,  A,  B).  In  the  urodeles  (newt 
and  Amblystoma)  the  ventral  lip  of  the  blastopore  remains 
stationary,  i.e.  it  retains  its  first  position,  and  the  anus  forms 
directly  from  its  posterior  end. 

The  Digestive  Tract  and  the  Gill-slits 

The  origin  of  the  archenteron  has  been  described  in  Chapter 
VI.  At  the  time  when  the  yolk -plug  is  drawn  in  from  the 
surface,  the  archenteron  has  begun  to  enlarge  (Fig.  26,  A). 
A  series  of  cross-sections  (Fig.  26,  B-E)  of  an  embryo  at  this 
stage  show  that  the  dorsal  and  lateral  walls  of  the  archenteron 
consist  of  a  single  layer  of  endodermal  cells,  while  the  floor  of 
the  archenteron  is  formed  by  the  upper  surface  of  the  yolk- 
mass.  The  uppermost  cells  of  the  yolk-mass  show,  to  some 
extent,  a  tendency  to  arrange  themselves  in  a  single  layer 
bounding  the  archenteron. 

Shortly  after  this  period  the  embryo  increases  in  length,  and 
the  archenteron  is  correspondingly  drawn  out  (Fig.  37).  The 
anterior  end  of  the  archenteron  enlarges,  and  the  yolk -mass  is 
pushed  posteriorly.  As  a  result  the  middle  and  posterior  parts 
of  the  archenteric  cavity  become  smaller  than  they  were  in  the 
earlier  stages  (Figs.  39,  40).  The  walls  of  the  anterior  portion 
of  the  archenteron  are  thin,  and  composed  of  a  single  layer  of 
cells.      A   blind   diverticulum   extending   from   this   enlarged 


Ch.  XIII]  ORGAXS  FROM   THE  EXDODERM  141 

anterior  portion  into  the  yolk-mass  behind  (Fig.  37,  A,  B) 
forms  the  beginning  of  the  liver. 

The  first  gill-slits  appear  at  a  stage  when  the  medullary  folds 
have  rolled  over  and  are  about  to  fuse.  At  the  present  stage, 
the  gill-slits  are  well  marked.  They  appear  along  the  lateral 
walls  of  the  enlarged  anterior  end  of  the  archenteron  as  solid 
outgrowths  of  its  wall.  At  the  posterior  end  of  the  archenteric 
cavity  the  position  of  the  blastopore,  which  has  now  closed, 
is  marked  by  a  diverticulum,  the  so-called  "  post-anal-gut " 
(Fig.  37).  It  is  in  this  region  that  the  neurenteric  canal  of 
the  embryo  persists  for  a  short  time  after  the  blastopore  has 
been  covered  over  by  the  medullary  folds.  The  pit-like  invagi- 
nation of  ectoderm,  the  proctodaeum,  has  opened  into  the  pos- 
tero-ventral  portion  of  the  archenteron  (Fig.  37,  B). 

At  the  time  when  the  tadpole  is  ready  to  emerge  from  the 
jelly-capsule  (Fig.  38),  the  anterior  portion  of  the  archen- 
teron has  become  larger  and  longer  (Fig.  39),  and  in  the  re- 
gion where  the  heart  forms,  ventral  to  the  pharynx,  an  inward 
projection  of  the  endodermal  wall  is  present.  In  the  middle 
region  of  the  embryo  the  lumen  of  the  archenteron  is  reduced 
to  a  small  cavity,  as  seen  in  cross-section  (Fig.  40),  and  is  now 
longer  from  above  downward  than  from  side  to  side.  The 
yolk-mass  as  a  whole  is  rounded  and  more  compact  than  in  the 
earlier  stages.  At  the  posterior  end  of  the  embryo  the  archen- 
teric cavity  bends  around  the  end  of  the  yolk-mass,  taking  a 
curved  course  to  open  on  the  ventro-posterior  surface  of  the 
body  by  the  anus. 

During  the  early  stages  of  development  the  cells  of  the  em- 
br3'o  have  been  exceedingly  active,  but  no  food  has  been  taken 
as  yet  into  the  digestive  tract,  for  the  mouth  does  not  open 
until  some  time  after  the  embryo  has  left  the  egg-membranes. 
All  the  cells  of  the  body  contain  yolk-granules,  which  serve  in 
part,  beyond  doubt,  to  supply  the  energy  necessary  for  develop- 
ment. A  large  amount  of  yolk  is  also  stored  up  in  the  endo- 
derm  cells  of  the  ventral  yolk-mass,  and  must  also  long  serve 
as  a  source  of  nourishment  for  the  young  tadpole. 

The  changes  in  shape  that  the  archenteron  passes  through 
seem  to  be  in  part  a  result  of  the  activity  of  the  endodermal 
cells,  and  in  part  the  necessary  result  of  the  change  in  shape 


142 


Ch.  XIII] 


ORGANS   FROM   THE   EXDODERM 


143 


that  the  whole  embryo  assumes.  The  early  enlargement  of 
the  anterior  archenteric  cavity  and  the  formation  of  a  single- 
layered  wall  at  the  anterior  end,  with  the  subsequent  formation 
of  the  gill-slits,  would  seem  to  be  the  result  of  the  activity  of 
the  endodermal  cells  of  those  regions.  On  the  other  hand, 
some  of  the  changes  in  shape  that  the  lumen  undergoes  would 
seem  to  be  due  to  the  change  in  shape  of  the  whole  embryo  as 
it  elongates  antero-posteriorly,  and  narrows  from  side  to  side. 
Nevertheless,  even  in  this  case  the  cells   do   not  seem   to  be 


Fig.  40.  —  Cross-section  through  the  middle  of  an  embryo  (3h  mm.).  AH.  Arehen- 
teron.  Ms.  Mesoblastic  somites.  X.  Xotochord.  Ns.  Xeural  crest.  M.  Medul- 
lary tube.  Pr.  Pronephros.  Sx.  Subnotochordal  rod.  So,  Sp.  Somatic  and 
splanchnic  mesoderm.     (After  Marshall.) 

entirely  passive,  for  the  number  of  cells  lining  certain  parts  of 
the  early  archenteron  is,  in  cross-section,  considerably  larger 
than  the  number  lining  the  same  region  at  a  later  stage. 
Either  certain  of  the  cells  have  pulled  away  from  the  surface 
and  have  passed  into  the  yolk,  or  else  they  have  changed  their 
position  relative  to  one  another  on  account  of  the  lengthening 
of  the  archenteron.  In  the  latter  case  the  total  number  of 
endoderm    cells    lininsf    the    archenteron   would    still   be   the 


144 


DEYELOPMEXT   OF   THE   FROG'S  EGG        [Cii.  XIII 


Crz. 


same   in   the  older  and  younger  embryos,  or   greater   in   the 

older  embryo  as  a  result  of  cell-division. 

The  first  three 
pairs  of  gill-slits 
appear  almost  si- 
multaneously; the 
first  two,  however, 
before  the  third. 
When  the  tadpole 
leaves  its  capside, 
there  are  five  pairs 
of  gill-slits ;  the 
two  new  pairs  have 
appeared  succes- 
sively behind  the 
third.  A  horizon- 
tal section  through 
the  larva  (Fig.  41) 
shows  to  best  ad- 
vantage the  five 
clefts  at  this  stage. 
"The  gill-pouches 
form  vertical  par- 
titions radiating 
outwards  from  the 
pharynx  to  the 
surface  -  ectoderm. 
Each  pouch  is 
formed  of  a  double 
fold  of  endoderm, 
the  two  layers  of 
which  are  in  close 
contact  with  each 
other.  The  outer 
ends    of    all    five 

pairs  of  gill-pouches  reach  the  ectoderm  and    fuse   with   its 

inner  or  nervous   layer.  "^     The  most  anterior  pouch  or  cleft 


Fig.  41.  — AR.  Archenteron.  BRi,  BR2,  BR3.  Branchial 
arches.  Hi,  H2,  m.  Gill-clefts.  HB.  Hyoid  cleft. 
HM.  Hyomandibular  cleft.  HY.  Hyoid  arch.  IN. 
Infundibulum.  OF.  Olfactory  pit.  OS.  Optic  stalk. 
P.  Pronephros.    S.  Segmental  duct.    (After  Marshall.) 


1  Marshall  ('93). 


Ch.  XIII]  ORGANS   FROM   THE   EXDODERM  I45 

is  the  hyomandibular  cleft,  and  this  is  followed  successively 
by  the  first,  second,  third,  and  fourth  branchial  clefts.  The 
last  is  the  smallest  and  is  often  imperfectly  developed  at  this 
time. 

The  visceral  or  gill-arches  lie  between  the  clefts.  The  first 
arch  between  the  hyomandibular  and  the  first  branchial  clefts 
is  the  hyoid  arch  (Fig.  41).  Then  follow  the  first  branchial 
arch  (BR^),  second  branchial  arch  (BR^),  and  third  branchial 
arch  (BR2).  Behind  the  fourth  branchial  pouch  there  is  an 
imperfectly  defined  fourth  branchial  arch. 

When  the  tadpole  leaves  its  jelly-capsule,  the  pouches  are 
still  double-walled,  solid  partitions  ;  but  about  the  time  when 
the  mouth  forms,  the  endodermal  lamellse  of  some  of  the 
pouches  separate  and  place  the  cavity  of  the  pharynx  in  com- 
munication with  the  exterior.  The  second  and  third  branchial 
clefts  open  first.  Later  the  first  branchial  cleft  opens,  and  later 
still  the  fourth. 

The  hyomandibular  cleft  is  at  first  like  the  others,  but  it  never 
opens  to  the  exterior.  After  its  formation  it  separates  from 
its  ectodermal  connection,  and  recedes  from  the  surface.  The 
lamellae  separate,  and  the  cleft  appears  as  a  diverticulum  of  the 
pharynx. 

Two  other  structures  arise  from  the  w^alls  of  the  pharynx 
shortly  before  the  hatching  of  the  tadpole.  "  The  lungs  arise 
as  a  pair  of  pouch-like  diverticula  of  the  w^alls  of  the  oesophagus. 
They  are  at  first  exceedingly  small  and  have  strongly  pigmented 
walls." 

The  thyroid  body  appears  about  the  time  of  hatching  as  a 
short  median  longitudinal  groove  along  the  wall  of  the  pharynx. 
"  The  groove  is  shallow  anteriorly,  but  deepens  at  the  hinder 
end,  Avhere  it  leads  into  a  small  conical  pit-like  depression  of 
the  endoderm,  forming  the  pharyngeal  floor,  just  in  front  of 
the  pericardial  cavity.  Soon  after  the  mouth  opens,  the  thyroid 
separates  completely  from  the  floor  of  the  pharynx,  remaining 
as  a  solid  rounded  mass  of  pigmented  cells,  in  close  contact 
with  the  anterior  wall  of  the  pericardium."^ 

1  Marshall  ('93). 


CHAPTER  XIV 

ORGANS  FROM  THE  MESODERM 

The  mesoderm  appears  as  a  distinct  layer  over  the  dorsal 
surface  of  the  embryo  at  the  time  when  the  dorsal  lip  of  the 
blastopore  is  moving  over  the  white  hemisphere  (Fig.  25). 
At  first  the  mesoderm  is  in  close  contact  with  the  endoderm, 
particularly  along  the  mid-dorsal  line.  The  notochord  soon 
separates  from  the  mesodermal  sheets  of  each  side  by  two  verti- 
cal furrows,  so  that  from  this  time  forward  there  are  two  lateral 
sheets  of  mesoderm,  separated  in  the  mid-dorsal  line  by  the 
notochord  (Fig.  26,  E).  Around  the  anterior  and  posterior 
ends  of  the  notochord,  the  two  sheets  of  mesoderm  are  con- 
tinued into  each  other. 

These  sheets  of  mesoderm  now  rapidly  extend  ventrally. 
This  down-growth  is  brought  about  by  additions  to  the  ven- 
tral borders  of  the  sheets.  The  new  cells  that  are  added 
come,  probably,  from  the  yolk-cells  along  the  free  borders  of 
the  mesoderm ;  the  yolk-cells  in  this  region  dividing  rapidly 
form  smaller  cells  that  are  joined  to  the  mesoderm. ^  At  the 
time  when  the  medullary  folds  appear  outlined  upon  the  sur- 
face, the  lateral  sheets  of  mesoderm  have  extended  ventrally 
and  to  a  certain  extent  have  fused  in  the  mid- ventral  line. 
The  cells  of  each  sheet  of  mesoderm  are  arranged  over  the 
greater  part  of  their  extent  into  two  layers ;  but  on  each  side 
of  the  notochord  the  mesoderm  is  somewhat  thickened  to  form 
the  beginning  of  the  segmental  plate  (Fig.  42) ;  and  in  this 
region  there  is,  in  the  early  stages  of  development,  no  distinct 
arrangement  of  the  cells  into  two  layers. 

1  According  to  some  authors  the  ventral  extension  of  mesoderm  results  from 
a  proliferation  of  the  mesoderm  that  is  first  laid  down  over  the  dorsal  region, 
but  it  seems  to  me  there  is  little  ground  for  such  an  assumption. 

14G 


Ch.  XI\^  organs   from   the   mesoderm  147 

Over  the  anterior  end  of  the  embryo  and  around  the  pharynx 
the  mesoderm  forms  a  thin  layer  of  cells,  loosely  held  together 
(Fig.  26,  B).  The  mesoderm  over  the  dorsal  surface  of  the 
pharynx  and  beneath  the  brain  plate  is  represented  by  only  a 
single  layer  of  somewhat  scattered  cells.  Around  the  blasto- 
pore there  is  a  thick  layer  of  mesodermal  cells  which  is  thickest 
on  the  dorsal  surface.  In  general,  in  the  posterior  region  of  the 
body  the  mesoderm  is  thicker  than  in  the  middle  and  anterior 
regions. 

The  Mesodermic  Somites 

In  the  following  stages  of  development  of  the  embryo  the 
dorsal  ectodermal  plate  is  lifted  up  and  rolled  in  to  form  the 
central  nervous  system  (Fig.  42).     The  mesoderm  lying  on 


Fig.  42.  — Cross-section  through  middle  of  embryo.  M.  Medullary  plate.  N.  Noto- 
chord.  Xc.  Neural  crest.  PS.  Primitive  segment-plate.  SO,  SP.  Somatic  and 
splanchnic  mesoderm. 

each  side  of  the  notochord  changes  shape  somcAvhat  during  this 
time.  It  forms  on  each  side  a  thick,  nearly  solid  mass  of  cells, 
the  plate  of  the  primitive  segments  or  segmental  plate  (Fig. 
42).  The  outermost  cells  of  this  mass,  i.e,  those  lying  nearest 
to  the  dorsal  surface,  now  show  a  tendency  to  arrange  them- 
selves into  an  epithelial  layer.  This  layer  is  at  first  continu- 
ous at  the  sides  with  the  outer  or  somatic  layer  of  cells  of 
the  lateral  mesodermal  sheets.  The  two  layers  of  cells  of  the 
lateral  mesodermal  sheets  (Fig.  42,  SO  and  SP),  the  somatic 
and  splanchnic  layers,  often  show  a  tendency  to  separate  and 
leave  a  cavity  between  them.     This  cavity  filled  with  fluid 


148 


DEVELOPMENT   OF   THE   FROG'S   EGG        [Ch.  XIV 


is  the  coelom,  or  body-cavity,  and  is  at  first  continued  into  tlie 
segmental  plate.  The  cavity  in  the  segmental  plate  lies  be- 
tween the  outer  epithelial  layer  and  the  inner  solid  mass  of 
cells. 

When  the  medullary  plate  of  the  embryo  begins  to  roll  in 
to  form  the  nerve-tube,  each  segmental  plate  begins  to  break 
up  transversely  into  a  series  of  blocks  or  mesodermic  somites. 

The  process  begins  first  in  the  region 
anterior  to  the  middle  of  the  embryo 
(Fig.  43).  The  mesodermic  somites 
are  at  first  somewhat  irregular  in  out- 
line. The  first  well-marked  somite  lies 
at  about  the  level  of  the  ganglion  of 
the  vagus  nerve.  In  front  of  this  there 
are  traces  of  another  somite  which  is 
partially  broken  up  into  loose  mesen- 
chymatous  tissue.  Still  further  for- 
ward, the  series  of  somites  is  replaced 
by  loose  mesenchyme.  In  the  frog  the 
number  of  head-somites  (or  structures 
Fig.  43.  -  Frontal  section  of    corresponding   to   them)  is   uncertain. 

Bombiuator.  (After Gotte.)       ^  ,      n      .      .i  ... 

MS.  Mesobiastic  somites.    At    first    the    primitive    segments    or 

fai  freT^'''''^'  ^^'  ^^''"  ^^^^^^^^  ^^'®  ^^^^  Separated  from  the 
lateral  sheets  of  mesoderm,  but  almost 
immediately  after  the  segmental  plate  has  begun  to  break 
up  transversely  into  somites,  these  begin  to  separate  also 
from  the  lateral  mesoderm.  This  separation  appears  first 
in  the  intersegmental  borders.  At  this  time  the  medullary 
folds  have  met  to  form  a  closed  tube.  Posterior  to  the 
fourth  segment,  the  segmental  plate  is  beginning  to  break 
up  into  blocks,  but  these  have,  as  yet,  no  sharply  marked 
outer  or  ventral  boundaries.  The  body-cavity  of  the  lateral 
mesodermal  sheet  is  at  first,  as  we  have  seen,  sometimes  con- 
tinued into  the  cavity  of  the  segmental  plate,  but  when  the 
constriction  of  the  plate  from  the  lateral  sheets  takes  place, 
this  communication  (the  communicating  canal)  is  lost.  Even 
in  the  younger  stages  there  is  a  differentiation  of  a  peripheral 
epithelial  layer  surrounding  the  dense  central  mass  or  kernel 
of  the  somites.     This  peripheral  part  is  represented  on  the 


Cn.  XIV] 


ORGANS   FROM   THE   MESODERM 


149 


outer  side  of  each  somite  by  the  entire  somatic  layer.  Along 
the  ventral  and  median  boundaries  of  the  somites  a  layer 
having  a  loose  epithelial  character  (mesenchyme)  is  also  to  be 
seen.  Thus  the  central  mass  which  is  to  develop  into  the 
myotome  lies  on  the  median  side  of  the  coelom,  and  is  wholly 
surrounded  by  an  epithelial  layer.  Frontal  sections  show  that 
this  layer  can  also  be  traced  inward  for  some  distance  between 
successive  somites  over  both  their  anterior  and  posterior  sur- 
faces (Fig.  44). 

"  Not  merely  is  mesenchyme  produced  by  the  thin  peripheral 
layer  of  the  somites,  but  in  anterior  regions  considerable  por- 
tions of  the  kernels  of  the  somites  also  undergo  a  metamor- 
phosis in  this  direction.  Thus,  if  I  be  not  mistaken,  a  somite 
immediately  in  front  of  somite  1  has  been  wholly  converted 
into  mesenchymatic  tissue.  The  kernel  of  the  succeeding  so- 
mite (somite  1)  has  given  rise  to  a  considerable  quantity  of 
mesenchyme,  and  the  process  has  been  manifested,  though  to  a 
less  degree,  even  in  succeeding  somites."^ 

At  the  time  when  fourteen  pairs  of  somites  are  present  ^ 
the  cells  of  the  more 
anterior  somites  have 
begun  to  differentiate 
into  muscle-fibres.  The 
cells  of  each  somite  elon- 
gate in  the  antero-pos- 
terior  direction  and 
become  cylindrical  in 
shape,  and  each  extends 
the  whole  length  of  its 
somite  (Fig.  44,  B). 
Each  cylindrical  cell  has 
at  first  but  a  single  nu- 
cleus. Around  the  wall 
of  the  cell  a  layer  of  fine 

fibrillse  appears.  The  original  nucleus  divides  and  re-divides 
into  many  nuclei,  which  lie  scattered  throughout  the  cell. 

1  Field  ('91). 

2  Four  days  after  fertilization  of  the  egg,  when  three  pairs  of  gills  have 
appeared. 


Fig.  44. — Frontal  sections  through  the  anterior 
end  of  Bombinator.  (After  Gotte.)  A.  Shows 
three  gill-pouches  (G),  and  mesoderm  of 
arches.  B.  Shows  formation  of  mesodermic 
somites  (MS).     PH.   Pharynx. 


150 


DEVELOPMENT   OF   THE   FROG'S   EGG        [Ch.  XIV 


The  development  of  the  musculature  of  the  head,  limbs,  and 
ventral  body-wall  takes  place  at  a  later  stage.  A  description 
of  the  origin  and  development  of  these  structures  is  beyond 
the  limit  of  the  present  account. 

The  Heart  and  Blood-vessels 

The  heart  appears  at  the  time  when  the  medullary  folds  have 
rolled  in,  and  have  met  along  the  mid-dorsal  line  ;  it  lies  below 
the  pharynx,  and  anterior  to  the  liver  (Fig.  37,  B).  The  meso- 
derm in  this  region  shows  a  tendency  to  split  into  two  sheets 
and,  where  the  heart  is  about  to  develop,  a  cavity,  a  part  of 


Fig.  45.  — Three  stages  in  development  of  heart.     E.  Endothelium.    PE.  Pericar- 
dium.   PH.  Pharynx.    W.  Wall  of  heart. 


the  coelom,  appears  between  the  sheets.  A  cross-section  of  the 
larva  (Fig.  45,  A)  shows  on  each  side  of  the  mid-ventral  line 
in  the  region  of  the  heart  the  somatic  and  splanchnic  layers 
widely  separated  from  each  other.  The  coelomic  cavities  of 
the  right  and  left  sides  are  not  continuous  across  the  middle 
line,  but  anterior  and  posterior  to  this  section  the  coelomic 
cavity  is  found  to  be  continuous  before  and  behind  with  the 
general  coelomic  space  on  each  side.  A  few  scattered  cells 
lie  in  the  middle  line  between  the  splanchnic  la3^er  and  the 
ventral  wall  of  the  pharynx  (Fig.  45,  A).     These  cells  have 


Ch.  XIV]  ORGANS  FROM  THE  MESODERM  151 

been  described  as  originating  from  the  ventral  wall  of  the  arch- 
enteron,  and  if  so,  have  had  a  different  origin  from  the  other 
cells  of  the  heart.  ^ 

At  a  somewhat  later  stage  of  development  the  walls  of  the 
coelomic  cavities  of  the  right  and  left  sides  separate  further 
(Fig.  45,  B).  The  splanchnic  layer  thickens,  and  begins  to  sur- 
round the  proliferation  of  scattered  "endodermal  cells."  These 
endodermal  cells  arrange  themselves  into  a  thin-walled  tube 
stretching  throughout  the  heart-region  (Fig.  45,  B).  Subse- 
quent development  shows  that  this  tube  becomes  the  endothe- 
lial lining  of  the  heart.  Around  this  endothelial  tube  the 
thickened  splanchnic  layers  now  begin  to  push  in  from  the  sides 
between  the  tube  and  the  lower  wall  of  the  pharynx.  The  tube 
becomes  finally  entirely  surrounded  by  mesoderm  (Fig.  45,  C). 
The  mesoderm  from  the  sides  that  has  met  beneath  the  pharynx 
forms  the  dorsal  mesentery  of  the  heart.  The  mesoderm  around 
the  tube  continues  to  thicken,  and  forms  later  the  musculature 
of  the  heart. 

At  first  the  heart  has  also  a  ventral  mesentery  formed  by  the 
union  of  the  walls  of  the  coelomic  cavities  below  it  (Fig.  45,  B), 
but  later  the  mesentery  is  in  part  absorbed  and  the  coelomic 
cavities  become  continuous  below  from  side  to  side,  forming  the 
pericardial  chamber.  The  outer  layer  of  somatic  mesoderm 
gives  rise  to  the  pericardium  itself. 

The  tubular  heart  is  attached  at  its  posterior  end  to  the 
liver  and  anteriorly  to  the  wall  of  the  pharynx.  It  becomes 
free  ventrally  and  later  also  dorsally  along  the  middle  of  its 
course,  and  owing  to  an  increase  in  length  is  bent  on  itself 
into  an  </)-shaped  tube  (Fig.  39). 

When  the  tadpole  is  4  J  mm.  in  length,  we  find  a  vessel  open- 
ing into  the  posterior  end  of  the  heart,  the  sinus  venosus, 
formed  by  the  union  of  two  large  vitelline  veins.  These  veins 
have  appeared  on  each  side  of  the  liver-diverticulum  and  con- 
tinue along  the  yolk-mass  in  a  fold  of  the  splanchnopleure. 
They  are  supposed  to  carry  to  the  heart  the  food-material  ab- 
sorbed from  the  yolk.     Into  the  sinus  venosus  empty  also  two 


1  At  least  these  cells  have  arisen  from  the  yolk-cells  after  the  ventral  meso- 
derm has  been  split  off. 


EF*    EF-^EF-^    EFl  EH 


EM 


B 


A  F"  AP-  TA    AF^ 


Fig.  46,  A.  — AF.  Afferent  branchial  vessel.  AR.  Anterior  cerebral  artery.  CA, 
CP.  Anterior  and  posterior  commissural  vessel.  EFi,  EF-,  EF-^,  EF^.  Efferent 
brancliial  vessels  of  the  first,  second,  third,  and  fourth  branchial  arches.  EH.  Ef- 
ferent hyoid  vessel.  EM.  Efferent  mandibular  vessel.  G.  Glomus.  O.  Aorta. 
P.  Pronephros.     RT.  Truncus  arteriosus.     S.  Segmental  duct.     (After  Marshall.) 

B.  —  AFi,  AF2,  AF3.  Afferent  branchial  vessels.  AU.  Auricle.  CV.  Cuvierian 
vein.  EFi,  EF^,  EF3,  EF^.  Efferent  branchial  vessels.  EH.  Efferent  hyoid 
vessel.  EM.  Efferent  mandibular  vessel.  G.  Glomus.  HV.  Hepatic  veins. 
MV.  Mandibular  vein.  MY.  Hyoidean  vein.  TA.  Truncus  arteriosus.  V.  Ven- 
tricle.    (After  Marshall.) 

162 


Ch.  xia^]  orgaxs  from  the  mesoderm  153 

veins  that  have  come  down  from  the  dorso-hiteral  region  of  the 
embryo.  These  are  the  Cuvierian  veins  formed  on  each  side 
by  the  union  of  the  posterior  and  anterior  cardinal  veins.  The 
posterior  cardinals  bring  back  the  blood  from  the  head-kidneys. 
Around  the  head-kidneys  these  veins  form  sinuses  that  are 
enormously  large.  Each  posterior  cardinal  also  receives  so- 
matic veins  from  the  posterior  part  of  the  body-wall.  The 
anterior  cardinal  veins  bring  back  blood  from  the  dorsal  part 
of  tlie  head-region. 

In  a  larva  4^  mm.  in  length,  the  blood-vessels  of  the  branchial 
region  have  also  appeared.  The  anterior  end  of  the  heart,  the 
truncus  arteriosus,  divides  into  a  right  and  left  branch,  which 
pass  forward  and  laterally  toward  the  base  of  the  gill-region.  In 
the  mandibular  arch  no  vessels  are  as  yet  present.  In  the  hyoid 
arch  an  irregular  space  appears  in  the  mesoderm.  In  the  first 
branchial  arches  two  vessels  appear,  a  large  efferent  vessel  (Fig. 
46,  for  an  older  embryo)  connected  with  the  dorsal  aorta,  and  a 
smaller  afferent  vessel.  The  latter  is  at  present  without  con- 
nection. In  the  second  branchial  arch  the  conditions  are  like 
those  in  the  first.  In  the  third  branchial  arch  only  a  small 
efferent  vessel  has  as  yet  appeared.  No  vessels  are  present 
at  this  time  in  the  fourth  branchial  arch.  The  dorsal  aorta  is 
represented  by  a  paired  vessel  in  the  dorso-pharyngeal  region. 
Opposite  the  hyoid  arch  each  branch  of  the  dorsal  aorta  di- 
vides into  a  dorsal  and  into  a  ventral  branch.  The  dorsal 
branches  meet  each  other  behind  the  infundibulum,  while  the 
ventral  branch  passes  forward  to  end  blindly  (Fig.  46).  The 
two  aortas  unite  posteriorly  into  a  single  vessel  at  the  level  of 
the  pronephros  (Fig.  46,  A). 

Tlie  condition  of  the  blood-vessels  shortly  after  the  tadpole 
has  left  its  envelopes  (it  is  then  7  mm.  in  length)  is  illustrated 
in  Figs.  46  and  47.  The  heart  has  enlarged  and  is  further 
twisted  on  itself.  The  aortic  bulb-portion  and  the  auricular 
and  ventricular  portions  are  distinctly  marked  from  each  other 
by  constrictions  of  the  tube.  The  right  and  left  branches  of 
the  aortic  bulb  have  grown  toward  the  gill-arches,  and  the 
afferent  vessels  of  the  first  and  second  branchial  arches  have 
united  with  the  ventral  aortic  branches  AF^  and  AF^.  The 
efferent  branches,  EF^  and  EF^,  of  the  first  and  second  bran- 


154 


DEVELOPMENT   OF   THE   FROG'S   EGG         [Ch.  XIV 


cliial  arches  have  greatly  enlarged,  and  the  efferent  and  afferent 
vessels  are  now  also  united  to  each  other  in  each  arch  by  small 
vessels  (Fig.  47)  or  capillary  tubes.  The  efferent  vessels  of 
these  two  arches  are  also  in  communication  with  the  dorsal 
aorta  of  their  respective  sides.  There  is  thus  established  at 
this  time  a  circulation  of  blood  from  the  heart  to  the  dorsal 
aorta  by  way  of  the  first  and  second  branchial  arches. 

In  the  third  and  fourth  branchial  arches  the  efferent  vessels 
have  appeared.     In  the  third  arch  the  beginning  of  an  affer- 


FiG.  47.  — AFi.  Afferent  branchial  vessel.  CV.  Anterior  cardinal  vein.  EF^.  Effer- 
ent branchial  vein.  G.  Pneumogastric  nerve.  JV.  Inferior  jugular  vein.  L. 
Capillary  loop  connecting  afferent  and  efferent  branchial  vessels.  N.  Notochord. 
O.  Aorta.  P.  Pericardium.  PH.  Pharynx.  SU.  Suckers.  V^.  Fourth  ven- 
tricle.    (After  Marshall.) 


ent  vessel  is  seen  (Fig.  46).  In  the  hyoid  arch  blood-vessels 
appear,  as  we  have  seen,  at  an  early  stage  of  development  and 
seem  to  correspond  to  those  in  the  branchial  arches,  but  after 
developing  to  a  certain  extent,  they  begin  to  degenerate.  In 
the  mandibular  arch  no  vessels  have  appeared  at  the  time  when 
the  larva  leaves  its  capsule.  Soon  after  this  time  a  vessel  de- 
velops in  this  arch,  and  a  small  diverticulum  arises  from  the 
dorsal  aorta  (Fig.  46,  B,  MV),  and  later  the  two  vessels  unite. 
The  origin  of  the  heart  has  been  described,  but  as  yet  the 


Ch.  XIV]  ORGANS   FROM   THE   MESODERM  155 

method  by  which  the  blood-vessels  are  formed  has  not  been 
fully  considered.  The  dorsal  aorta  is  the  first  vessel  to  arise. 
A  series  of  isolated  lacunae  appear  in  the  mesoderm  along  the 
roof  of  the  pharynx,  and  by  opening  into  one  another  form 
a  pair  of  longitudinal  vessels.  Vessels  next  appear  in  the 
first  and  second  branchial  arches.  Similar  vessels  arise  later 
in  the  third  and  fourth  branchial  arches.  In  the  hyoid  and 
mandibular  arches  the  vessels  appear,  as  we  have  seen,  later 
still.  These  branchial  blood-vessels  originate  in  part  as  iso- 
lated lacunse  in  the  mesoderm,  and  in  part  as  outgrowths  of 
already  existing  vessels.  For  instance,  lacunar  vessels  appear 
in  the  mesoderm  of  the  gill-arches,  two  in  each  arch.  One 
of  these  is  the  efferent  lacunar  vessel,  and  later  connects  with 
a  corresponding  diverticulum  from  the  dorsal  aorta,  and  the 
other  lacunar  vessel  is  the  afferent  vessel  of  the  same  arch. 
This  latter  vessel  grows  ventrally  toward  the  diverticulum 
from  the  truncus  arteriosus  and  unites  with  it. 

The  walls  of  the  blood-vessels  are  formed  directly  from  the 
mesodermal  cells  around  the  lacunae.  "The  blood-corpuscles 
are  free  cells  that  have  been  left  in  the  lacuna-spaces,  or  more 
usually  are  cells  budded  off  at  a  later  stage  from  the  walls  of 
the  vessels  into  their  cavities."-^  At  first  the  blood-corpuscles 
are  simply  spherical  cells  containing  yolk-granules.  Only  after 
the  embrj^o  is  hatched  do  many  of  the  corpuscles  begin  to 
acquire  the  shape  and  character  of  red  blood-corpuscles. 

The  Pronephros 

The  excretory  system  of  the  young  embryo  is  represented  on 
each  side  by  the  pronephros  and  the  segmental  duct.  Whether 
the  pronephros  and  duct  arise  in  part  from  an  early  ingrowth 
of  ectoderm  or  whether  they  develop  in  situ  from  the  somatic 
mesoderm  is  perhaps  still  open  to  doubt.  Field  ('91),  who  has 
worked  out  most  recently  the  development  of  the  pronephros 
and  segmental  duct  in  the  frog,  describes  the  organ  as  coming 
entirely  from  the  mesoderm.  We  shall  follow  closely  Field's  ac- 
count.    The  pronephros  appears  at  a  stage  when  the  medullary 

1  Marshall  ('93). 


156        DEVELOPMENT  OF  THE  FROG'S  EGG    [Ch.  XIV 

plate  is  first  formed.  It  is  well  marked  at  the  time  when  the 
medullary  folds  have  rolled  in,  but  have  not  yet  fused.  A 
thickening  of  the  somatic  layer  of  the  lateral  mesoderm  near  the 
second  mesoblastic  somite  marks  the  beginning  of  the  prone- 
phros (Fig.  48,  A).  At  a  later  stage,  the  mesodermic  thick- 
ening becomes  larger,  and  the  anterior  end  arches  over  toward 
the  coelomic  cavity^  to  form  the  first  nephrostome.  The  ventro- 
posterior  part  of  the  nephrostomal  thickening  is  continued 
backward  as  a  thickening  of  the  somatic  wall  as  far  as  the 
seventh  somite,  to  form  the  segmental  duct.  A  canalization 
now  takes  place  in  the  nephrostomal  portion  and  in  the  seg- 
mental duct.  Three  short  tubes  or  canals  appear  in  the 
pronephric  mass  running  outward  from  the  coelom  (Fig.  41). 
Constrictions  appear  between  the  first  and  second,  and  between 
the  second  and  third  canalized  tracts  (Fig.  48,  B),  and  short 


A  B  C 

Fig.  48.  —  Three  stages  in  the  formation  of  the  pronephros.    (A  and  C  after  Field.) 

hollow  stalks  are  formed  leading  ventrally  into  the  longitu- 
dinal canal  of  the  segmental  duct. 

A  proliferation  of  cells  from  the  somatic  layer  of  the  meso- 
blastic somites,  dorsal  to  the, pronephros,  gives  rise  to  a  cover- 
ing of  mesoderm  for  the  pronephros,  the  pronephric  capside. 
A  little  later  a  protrusion  of  the  splanchnic  wall  opposite  to 
the  funnels  of  the  pronephros  forms  the  glomus  (Fig.  47,  B). 
The  glomus  becomes  filled  with  blood,  and  seems  to  have  a 
direct  connection  with  the  dorsal  aorta.  The  bulging  portion 
of  the  glomus  protrudes  into  the  coelom,  and  its  cavity  is  sepa- 
rated from  the  coelomic  cavity  by  only  a  single  layer  of  cells. 

At  the  time  when  the  embryo  is  hatched,  the  duct  of  the 
pronephros,  the  segmental  duct,  has  fused  with  the  wall  of 
the  cloaca,  and  the  lumen  of  the  duct  opens  into  the  digestive 


Ch.  XIV]  ORGANS  FROM   THE   MESODERM  I57 

tract  (Fig.  41).  Presumably  the  pronephros  is  functionally 
active  at  this  time.  The  arrangement  of  the  tubes  of  the 
pronephros,  and  their  relation  to  the  common  tube  or  prone- 
phric  duct,  is  shown  in  Fig.  48,  C.  The  three  nephrostomes 
open  into  three  collecting  tubules,  and  these  tubules  have 
elongated  independently  of  one  another.  The  first  collecting 
tubule  is  short ;  the  second  is  thrown  into  several  turns  and 
opens  into  the  pronephric  duct  a  short  distance  from  the  first. 
The  collecting  tubule  from  the  third  nephrostome  opens  some 
distance  behind  the  point  of  opening  of  the  second.  The  seg- 
mental duct  is  thrown  into  a  series  of  turns  between  the  first 
and  second  collecting  tubules  ;  and  as  it  leaves  the  pronephric 
region  it  takes  at  first  a  tortuous  course,  and  then  runs  as  a 
straight  tube  backward  to  the  cloacal  opening. 

The  posterior  cardinal  veins  have  appeared  at  this  time,  and 
in  the  region  of  the  head-kidneys  these  veins  widen  into  a  sinus 
lying  amongst  the  windings  of  the  collecting  tubules  of  the 
pronephric  duct.  The  glomus  of  each  side  reaching  from  the 
region  of  the  first  to  that  of  the  third  nephrostome,  and  lying 
exactly  opposite  the  nephrostomes,  is  well  developed  (Fig.  46). 

So  far  the  description  of  the  development  of  the  excretory 
system  has  been  that  given  by  Field.  The  same  author  adds  : 
"According  to  the  account  which  at  present  receives  the  most 
general  acceptance,  the  pronephros  first  appears  as  an  outfold- 
ing  of  the  somatopleure  in  the  form  of  a  longitudinal  groove. 
The  anterior  end  of  this  groove  is  destined  to  become  the  prone- 
phros, the  remaining  portion  is  constricted  off  to  form  the  seg- 
mental duct.  Since  the  process  of  constriction  advances  from 
before  backward,  stages  may  be  found  in  which  a  completed 
tube  is  continuous  posteriorly  with  a  mere  groove  of  the  soma- 
topleure. In  the  anterior  region  the  groove  remains  in  com- 
munication with  the  body-cavity,  and  grows  down  toward  the 
ventral  surface  of  the  embryo  in  the  form  of  a  broad  pocket. 
The  slit-like  peritoneal  opening  of  this  pouch  closes  through- 
out the  greater  part  of  its  length,  leaving,  however,  two  or 
three  regions  of  incomplete  closure,  the  fundaments  of  the 
nephrostomes." 

'•  The  nephrostomal  tubules  are  formed  by  the  fusion  of  the 
walls  of  the  pouch  between  two  nephrostomes.     The  regions  of 


158  DEVELOPMENT   OF   THE   FROG'S   EGG        [Cii.  XIV 

fusion  extend  in  vertical  lines  from  the  nephrostomal  margin 
of  the  pouch  nearly  to  its  ventral  border,  where  there  is  left 
an  unfused  and  therefore  continuous  longitudinal  tract  con- 
stituting the  canal  which  I  have  called  the  collecting  trunk. "  ^ 
Field  continues,  "In  opposition  to  this  view,  I  would  maintain : 
(1)  That  the  first  trace  of  the  excretory  system  consists  of  a 
solid  proliferation  of  somatopleure,  the  pronephric  thicken- 
ing; (2)  that  the  lumen  of  the  system  arises  secondarily;  and 
(3)  that  the  pronephric  tubules  do  not  appear  in  consequence  of 
the  local  fusion  of  the  walls  of  a  widely  open  pouch,  but  that 
they  are  differentiated  at  an  early  stage  from  the  hitherto 
indifferent  pronephric  thickening." 

The  pronephric  duct  of  the  Amphibia  arises,  according  to 
one  view,  as  we  have  seen  above,  from  an  evagination  of  soma- 
topleure^ its  lumen  being  therefore  a  detached  portion  of  the 
body-cavity.  A  second  view  of  the  origin  of  the  duct  is,  that 
it  arises  from  a  solid  proliferation  of  somatopleure.  Field 
agrees  with  the  latter  view.  A  third  view  maintains  that  the 
duct  is  ectodermic  in  origin.  Field  has  shown,  however,  that 
in  the  Amphibia  the  excretory  system  develops  most  probably 
without  any  participation  of  the  ectoderm  in  its  formation. 

i"This  view  of  the  development  of  the  pronephros,  although  suggested  by 
Wilh.  Miiller,  was  first  described  in  detail  by  Goette  for  Bombinator,  and  was 
later  extended  to  other  Amphibia  by  the  researches  of  Fiirbringer.  It  has  been 
entirely  confirmed  by  Wichmann,  by  Hoffmann,  and  still  more  recently  by 
Marshall  and  Bles."     (Field,  '91,  page  281.) 


CHAPTER   XV 

ORGANS  FROM  THE   ECTODERM 

The  outer  covering-layer  of  the  embryo,  the  ectoderm,  gives 
rise  to  the  nervous  system  and  organs  of  special  sense  (eyes, 
ears,  nose).  The  adhesive  glands  or  "suckers"  are  also  formed 
by  this  la^^er ;  and  the  anterior  and  posterior  divisions  of  the 
digestive  tract,  the  stomodeeum  and  proctodaeum,  have  a  lining 
of  ectoderm. 

In  the  present  chapter  we  shall  follow  the  development  of 
these  organs. 

The  Central  Nervous  System 

The  medullary  plate  appears  on  the  surface  of  the  young 
embryo  at  the  time  when  the  yolk-plug  is  about  to  be  drawn 
in  from  the  surface.  It  extends  over  about  one-third  of  the 
circumference  of  the  egg  and  is,  at  first,  quite  broad.  It  is 
slowly  converted  into  a  tube  by  the  drawing  together  of  its 
material,  and  by  a  subsequent  over-rolling  of  its  sides  to  meet 
in  the  mid-dorsal  line.  This  change  into  a  furrow,  and  then  into 
a  closed  tube,  involves  extensive  movements  of  the  material  of 
the  plate.  Whether  the  plate  moves  as  a  whole,  or  whether 
the  movement  is  only  the  sum-total  of  changes  in  shape  and 
position  of  the  individual  cells,  is  not  known  (compare  Figs.  26, 
42,  50).  While  the  medullary  tube  is  developing,  the  embryo 
as  a  whole  is  changing  its  spherical  shape  into  a  more  elon- 
gated form  and  the  medullary  tube  is  also  drawn  out. 

The  medullary  plate  is  formed,  for  the  most  part,  from  a 
thickening  of  the  inner  layer  of  the  ectoderm  (Figs.  26  and 
42).  It  is  continuous  on  each  side  with  a  broad  flange  or  ridge 
of  thickened  ectoderm  (Fig.  42,  Nc).  This  ridge  of  cells,  the 
neural  crest  or  ridge,  is  also  lifted  up  during  the  formation  of 

159 


160  DEVELOPMENT   OF   THE   FROG'S   EGG  [Cii.  XV 

the  tube,  forming  a  broad  sheet  of  cells  on  each  side,  continu- 
ous with  the  dorsal  edges  of  the  closing  tube.  These  lateral 
sheets  are  very  large  and  conspicuous  at  the  anterior  end  of 
the  nerve-tube.  The  subsequent  history  of  these  structures 
will  be  followed  later. 

The  first  part  of  the  medullary  tube  to  close,  is  the  antero- 
median portion,  and  from  this  point  the  closure  of  the  tube 
extends  anteriorly  and  posteriorly.  At  the  anterior  end,  the 
tube  remains  open  latest ;  at  the  posterior  end,  the  medullary 
folds  arch  over  the  blastopore,  as  already  described. 

When  the  medullary  folds  have  met  along  the  mid-dorsal 
line,  the  apposed  edges  fuse,  and  the  outer  layer  of  ectoderm 
then  becomes  continuous  over  the  outer  surface  of  the  embryo. 
A  part  of  the  same  layer  has  been  cut  off  and  lines  the  cavity 
of  the  neural  tube.  The  nerve-tube  soon  loses  all  connection 
with  the  overlying  ectoderm  (Fig.  40). 

The  anterior  end  of  the  nerve  tube  is  larger  than  the  rest, 
and  this  end  is  at  first  bent  down  nearly  at  right  angles  to  the 
long  axis  of  the  more  posterior  portion  (Fig.  37,  A).  The 
bending  begins  at  the  front  end  of  the  notochord.  A  slight 
transverse  infolding  of  the  Avail  of  the  anterior  end  of  the  tube 
takes  place  soon  after  its  closure,  and  later  another  transverse 
infolding  occurs,  still  further  forward.  As  a  result  three  divi- 
sions or  vesicles  of  this  region  are  produced.  They  correspond 
to  the  fore-brain,  mid-brain,  and  hind-brain  respectively.  The 
fore-brain  (Fig.  37,  Fb)  is  the  large  anterior  vesicle.  From  it 
develops  later  the  third  ventricle,  the  pineal  body,  the  infun- 
dibulum,  the  optic  vesicles,  and  the  cerebral  hemispheres.  The 
mid-brain  (Fig.  37,  B)  is  the  smallest  of  the  three  divisions,  and 
gives  rise  to  the  optic  lobes  and  to  the  Sylvian  aqueduct.  The 
hind-brain  is  continued  into  the  more  posterior  medullary  tube. 
It  lies  in  the  same  plane  with  the  medullary  tube,  and  repre- 
sents only  a  somewhat  enlarged  part  of  the  tube.  The  hind- 
brain  becomes  the  medulla  oblongata,  and  from  its  roof  the 
cerebellum  is  formed. 

The  roof  of  the  fore-brain  is  very  thin.  Near  the  middle  of 
its  upper  margin  an  evagination  is  formed,  which  is,  at  first, 
only  a  hollow  diverticulum  (Fig.  37,  B),  but  when  the  tadpole 
leaves  its  capsule,  the  peripheral  end  of  the  outgrowth  forms  a 


Ch.  XV]       ORGANS  FROM  THE  ECTODERM         161 

small  round  knob.  This  knob,  the  pineal  body,  lies  just  below 
the  surface-ectoderm.  Later  the  structure  grows  forward,  and 
becomes  dilated  at  its  distal  end.  The  dilated  end  or  bulb  re- 
mains connected  with  the  brain  by  a  stalk.  White  particles 
develop  in  the  bulb,  so  that  it  stands  out  in  strong  contrast  to 
the  dark  surface  of  the  brain. 

At  the  time  of  closure  of  the  medullary  folds,  a  mid-ventral 
diverticulum  forms  from  the  floor  of  the  fore-brain.  This  is 
the  infundibulum.  It  is  in  close  contact  with  the  anterior  end 
of  the  notochord  (Fig.  38).  The  infundibulum  is  throughout 
its  subsequent  history  a  wide  sac  with  thin  walls.  It  soon 
comes  into  close  connection  with  another  structure,  the  pitui- 
tary body  (Fig.  39).  The  pituitary  body  arises  very  early, 
even  before  the  neural  tube  is  closed,  as  a  solid  ingrowth,  or 
cord  of  cells,  from  the  ectoderm,  immediately  in  front  of  the 
anterior  end  of  the  medullary  plate  (Fig.  37,  A).  Later,  this 
small,  solid,  tongue-like  process  projects  inward  from  the 
ectoderm  beneath  the  brain  and  above  the  dorsal  wall  of  the 
pharynx.  The  inner  end  of  the  ingrowth  expands  into  a  flat- 
tened mass  of  cells,  which  lies  immediately  beneath  the  anterior 
end  of  the  notochord.  This  mass  becomes  later  the  pituitary 
body,  while  the  rest  of  the  process  forms  a  slender  stalk  con- 
nected at  one  end  with  the  ectoderm. 

The  Eyes 

The  eyes  develop  in  part  from  the  walls  of  the  fore-brain. 
Even  before  the  neural  tube  is  closed,  in  the  embryos  of  some 
species  of  frogs,  two  pigmented  areas  may  be  seen  on  the  an- 
tero-lateral  walls  at  the  anterior  end  of  the  infolding  medullary 
plate.  These  pigmented  areas  mark  the  region  from  which  a 
pair  of  evaginations  of  the  fore-brain  will  develop  to  form  the 
optic  vesicles.  The  hollow  vesicles  push  out  laterally  toward 
the  sides  of  the  head.  Each  tubular  evagination  then  becomes 
constricted,  forming  a  distal  hollow  bulb  and  a  proximal  hollow 
stalk  (Fig.  49).  The  bulb  gives  rise  to  the  retina  and  to  the 
pigment  behind  the  retina,  while,  according  to  Marshall  ('93), 
the  stalk  forms  a  path  along  which  the  fibres  of  the  optic 
nerve  pass  from  the  eye  to  the  brain.  The  outer  hemisphere 
of  the  optic  bulb  flattens  and  then  pushes  in  so  that  the  former 


162 


DEVELOPMENT   OF   THE   FROG'S  EGG  [Cii.  XV 


cavity  of  the  vesicle  is  nearly  obliterated  (Fig.  49)  ;  and  at 
the  same  time  the  inturned  wall  becomes  greatly  thickened. 
There  is  thus  formed  an  open,  cup-shaped  structure  with  the 

opening  of  the  cup  turned 
outward.  The  wall  of  this 
optic  cup  lying  toward  the 
brain  remains  thin,  and  pig- 
ment soon  appears  in  it. 
The  inturned  wall  becomes 
the  retina  of  the  eye. 

At  the  time  when  the 
optic  bulb  turns  in  on  it- 
self, a  thickening  of  the 
inner  layer  of  ectoderm  op- 
posite the  optic  cup  takes 
place.  This  thickening 
forms  a  solid  mass  of  cells 
projecting  into  the  open 
mouth  of  the  cup.  It  be- 
comes hollow  and  then  sep- 
arates from  the  ectoderm 
(Fig.  49),  filling  up  the 
opening  of  the  optic  cup, 
and  forms  later  the  lens  of 
the  eye.  In  the  space  left  between  the  lens  and  the  retinal 
layer  the  vitreous  body  of  the  eye  forms.  The  later  stages  of 
the  development  of  the  eye  take  place  after  the  embryo  leaves 
its  capsule.  The  nerve-fibres  that  develop  from  the  retina 
and  pass  into  the  brain  along  the  optic  stalks  have  not  yet 
appeared. 

The  Ears 

While  the  neural  groove  is  closing,  a  pair  of  thickened  circu- 
lar patches  of  the  inner  layer  of  the  ectoderm  arises,  one  on 
each  side  of  the  head  near  the  hind-brain.  After  the  closure 
of  the  neural  tube  each  area  forms  a  shallow  pit  with  the  con- 
cavity turned  outward,  and  each  is  covered  by  the  outer  layer 
of  the  ectoderm.  The  pit  deepens,  the  outer  edges  come 
together,  and  a  hollow  vesicle  is  formed   before  the  tadpole 


Fig.  49. — Cross-section  through  head  and 
eyes.  F.  Fore-brain.  L.  Lens  of  eye. 
OP.  Optic  cup.  OS.  Optic  stalk.  PH. 
Pharynx.  PT.  Pituitary  body.  ST.  Sto- 
modaeum. 


Ch.  XV]      ORGANS  FROM  THE  ECTODERM  163 

leaves  the  capsule.     These  auditory  vesicles  separate  from  the 
surface  ectoderm.     "At  the  time  of  the  separation  the  vesicle 
is   a   closed   sac  somewhat  pyriform   in   shape;    its   lower  or 
ventral  portion  being  spheri- 
cal  and    lying   opposite   the 
notochord,    and     its     dorsal 
wall    being    j)rolonged     up- 
wards   into    a    short    blind 
diverticulum    lying    at    the 
side  of  the  hind-brain.     The 
wall   of  the  vesicle  consists 
of  a  single  layer  of  cubical 
or    columnar    cells."      This 


-tsiciniic^ 


ectodermal  sac  becomes  the  Fig.  so.  —  Cross-section  through  hind- 
sensory  lining  of  the  inner  ^o^^^^i  ^""^  '°°''  '"  ^^^*  ^' 
ear  (Fig.  50). 

The  Nerves 

At  the  time  when  the  medullary  plate  forms  as  a  thickening 
of  the  ectoderm,  there  also  forms,  as  we  have  seen,  on  each  side 
of  the  plate  a  lateral  7ieural  ridge  or  jylate  of  ectoderm.  Each 
neural  ridge  appears  at  first  as  a  continuation  of  one  side  of 
the  thickened  medullary  plate  (Fig.  26).  A  slight  constric- 
tion on  each  side  marks  the  line  of  demarcation  between  the 
medullary  plate  and  the  neural  ridge  (Fig.  42).  The  neural 
ridges  are  more  conspicuous  at  the  anterior  end  of  the  medul- 
lary plate ;  they  also  develop  somewhat  earlier  in  this  region. 
After  the  medullary  plate  has  rolled  up  to  form  the  medullary 
tube,  the  lateral  neural  ridges  are  also  carried  up,  retaining  for 
a  time  their  primitive  connection  with  the  outer  (now  dorsal) 
part  of  the  medullary  tube  (Fig.  40). 

The  neural  ridges  next  become  broken  up  into  a  series  of 
dorsal  nerves,  the  cells  collecting  at  certain  regions,  and  thin- 
ning out  and  disappearing  in  the  intermediate  regions.  The 
dorsal  nerves  grow  down  later  between  the  myotomes  and  the 
nerve-cord.  Accumulations  of  cells  occur  at  certain  regions 
on  each  dorsal  nerve  to  form  the  ganglion  of  the  dorsal  root, 
and  nerve-fibres  are  spun  out  from  the  cells  of  the  ganglion. 
The    ventral   roots  of   the  spinal   nerves  appear  much   later. 


164  DEYELOPMEXT   OF   THE   FROG'S   EGG  [Cii.  XV 

Marshall  ('93)  says  the  cranial  nerves,  ''  which  are  uncloiiljtedly 
derived  from  the  neural  ridges,  are  the  trigeminal,  the  facial 
and  auditory,  and  the  sensory  branches  of  the  glosso-pharyn- 
geal  and  pneumogastric  nerves."  These  nerves,  "although 
arising  from  the  neural  ridges  in  the  same  way  as  the  dorsal 
roots  of  the  spinal  nerves,  yet  differ  from  these,  and  agree 
amongst  themselves  in  certain  important  features." 

"  I.  The  nerves  in  question,  in  place  of  growing  downwards 
like  the  spinal  nerves,  alongside  the  central  nervous  system, 
grow  outwards  close  to  the  surface  of  the  embryo  between  the 
epiblast  and  the  mesoblast." 

"  II.  Each  of  these  four  nerves  acquires  a  new  connection 
with  the  surface  epiblast  some  considerable  distance  beyond 
the  root  of  origin  from  the  brain,  and  at  about  the  horizontal 
level  of  the  notochord ;  at  this  place  and  at  any  rate  in  part 
from  the  surface  epiblast  itself,  the  ganglion  of  the  nerve  is 
formed." 

''  III.  The  nerves  have  special  relations  to  the  gill-slits,  each 
nerve  dividing  into  two  main  branches,  which  embrace  between 
them  one  of  the  gill-slits." 

"  IV.  A  special  system  of  cutaneous  nerves  is  developed 
from  the  surface  epiblast  in  connection  with  these  four  nerves, 
forming  the  lateral  line  system  of  nerves." 

The  pneumogastric  nerves  are  "wing-like"  expansions  of  the 
neural  plate,  extending  more  than  half-way  down  the  side  of 
the  pharjmx.  At  the  time  when  the  larva  leaves  the  capsule, 
a  thickening  of  the  ectoderm  on  each  side  opposite  this  nerve 
and  at  the  level  of  the  notochord  develops,  and  fuses  with  the 
nerve.  From  this  double  origin  arises  the  ganglion  of  the 
pneumogastric.  A  lateral  line  thickening  has  appeared  as  a 
solid  cord  of  cells  on  each  side,  extending  from  the  pneumo- 
gastric backward  along  the  side  of  the  embryo. 

It  is  not  possible  to  enter  here  into  the  details  of  the  develop- 
ment of  the  other  cranial  nerves  enumerated  above.  The 
development  of  the  first,  third,  fourth,  and  sixth  nerves  has 
not  as  yet  been  fully  worked  out.  The  origin  of  the  optic 
nerve  has  been  described  in  connection  with  the  development 
of  the  eye. 


Ch.  XV] 


ORGANS  FROM  THE  ECTODERM 


165 


The  Appearance  of  Cilia  on  the  Surface  of  the 

Embryo 

If  the  living  embryo  be  examined  at  the  time  when  the 
neural  folds  have  appeared,  it  will  be  seen  that  the  embryo 
slowly  rotates  within  the  jelly-capsule.  This  rotation  is  the 
result  of  the  activity  of  certain  ciliated  ectodermal  cells.  The 
distribution  of  these  cells  over  the  surface  of  the  body  has  been 
recently  described  by  Assheton  ('96).  Assheton  states  that 
at  the  time  when  the  medullary  folds  are  first  visible,  and  even 
after  they  have  begun  to  roll  in,  there  are  no  traces  of  cilia  on 


Fig.  51.  —  Embryo  of  Rana.    The  arrows  show  the  direction  of  currents  of  water 
over  the  surface.    A.  Side  view.    B.  Ventral  view.     (After  Assheton.) 


the  surface  of  the  embryo.  Before  the  neural  folds  have  met 
in  the  middle  line  the  ectoderm  has  become  ciliated  in  certain 
regions,  as  can  be  demonstrated  by  the  streaming  movement 
of  granules  of  carmine  placed  on  the  surface.  The  arrows  in 
Fig.  51  show  the  direction  of  the  flow  of  granules  over  the  sur- 
face. The  lateral  edges  of  the  anterior  end  of  the  medullary 
folds  seem  to  show  the  first  traces  of  cilia,  and  a  few  hours 
later  (Fig.  51,  A)  cilia  have  also  appeared  along  the  sides  of 
the  folds. 


166  DEVELOPMENT   OF   THE  FROG'S   EGG  [Ch.  XV 

•  As  the  medullary  folds  grow  nearer  together,  the  ciliation  ex- 
tends further  back  along  the  sides  of  the  dorsal  surface.  When 
the  folds  have  just  touched  at  the  anterior  end,  cilia  appear 
on  the  antero-ventral  surface  of  the  embryo,  in  the  region  where 
the  mouth  subsequently  forms.  The  direction  of  the  currents 
set  up  is  from  before  backward.  The  whole  of  the  dorsal 
surface  next  becomes  ciliated.  The  ciliation  spreads  rapidly 
and  at  the  time  when  seven  or  eight  mesoblastic  somites  are 
present  (when  the  embryo  is  3  mm.  in  length)  it  has  extended 
over  the  whole  surface  of  the  embryo.  The  currents,  however, 
differ  very  much  in  intensity.  Figure  51,  A,  B,  shows  the 
direction  of  the  flow,  the  larger  arrows  indicating  stronger  cur- 
rents. The  action  of  the  cilia  is  strongest  at  the  anterior  end 
of  the  body.  A  well-defined  stream  passes  over  the  bases  of 
the  gills,  which  have  begun  to  appear  at  this  time.  Over  the 
ventral  surface  the  currents  move  slowly  and  in  eddies.  At 
the  hinder  end  of  the  embryo  the  action  of  the  cilia  directs  the 
currents  of  water  toward  the  blastopore  and  anus.  When  the 
embryo  measures  4  mm.,  the  so-called  "suckers"  have  appeared, 
and  the  currents  in  that  region  have  changed  their  direction. 
These  "  suckers "  are  in  reality  mucous  glands  that  secrete  a 
sticky  substance  by  means  of  which  the  embryo  can  fix  itself 
to  objects  with  which  it  comes  in  contact.  The  edges  of  the 
glands  have  well-developed  cilia,  which  direct  a  stream  of 
water  over  the  stomodseal  depression,  and  thence  backward 
between  the  glands  (Fig.  51,  B).  In  older  embryos,  when 
the  glands  have  united  to  each  other  in  the  mid- ventral  line, 
the  direction  of  the  currents  in  this  region  is  altered.  The 
central  stream  now  turns  outward  around  the  anterior  sides 
of  the  glands  and  passes  backward  along  the  sides.  In  older 
larvae  (8  mm.)  small  special  currents  run  over  the  edges  of 
the  adhesive  glands  and  into  the  depressions  within  the 
glands. 

The  cilia  that  cause  the  flow  of  water  over  the  surface 
of  the  embryo  are  not  developed  by  all  the  ectodermal  cells. 
Even  where  the  currents  are  most  active,  the  cilia-bearing 
cells  are  slightly  less  abundant  than  the  non-ciliated  cells. 
Each  ciliated  cell  bears  on  its  outer  surface  numerous  short 
cilia. 


Ch.  XV]  ORGANS   FROM   THE   ECTODERM  167 

The  gill-filaments  also  carry  cilia  in  the  proportion  of  one 
ciliated  cell  to  two  non-ciliated  cells.  The  effect  of  the  cilia 
becomes  less  conspicuous  after  the  larvae  have  reached  7  or  8 
mm.  in  length,  although  even  in  much  later  stages  the  cilia  are 
still  found  over  all  parts  of  the  body.  Their  motion  is  suffi- 
ciently strong  to  cause  the  embryo  (6  or  7  mm.  in  length)  to 
move  forward,  if  placed  on  a  glass  plate,  at  the  rate  of  one 
millimetre  in  from  four  to  seven  seconds. 


CHAPTER   XVI 

EFFECTS   OF   TEMPERATURE  AND   OF  LIGHT  ON 
DEVELOPMENT 

It  has  been  long  known  that  the  rate  of  development  within 
certain  limits  is  dependent  on  temperature.  The  development 
of  the  frog's  Qg^  is  very  much  retarded,  or  even  stopped,  in 
water  at  the  freezing-point.  In  North  America,  Rana  tempo- 
raria  often  lays  its  eggs  so  early  in  the  spring  that  the  water 
is  afterward  frozen.  The  eggs  that  are  caught  in  the  ice  are 
generally  killed,  but  those  that  lie  in  the  water  below  often 
remain  alive  and  will  subsequently  develop  normally. 

Hertwig  ('94)  has  shown  that  the  maximum  temperature 
for  normal  development  of  the  eggs  of  Rana  fusca  is  about 
25  degrees  C.  Eggs  develop  very  rapidly  at  this  tempera- 
ture, and  in  twenty-four  hours  have  reached  a  stage  of  ad- 
vancement corresponding  to  that  at  the  end  of  the  second  day 
for  the  average  temperature  of  16  degrees.  A  temperature 
of  25  to  30  degrees  C.  long  continued,  or  a  temperature  of  30 
to  35  degrees  for  a  short  time,  injures  the  eggs  ;  their  develop- 
ment is  arrested  and  many  die.  Eggs  that  have  been  partially 
injured  by  heat  (after  two  or  three  hours  at  30  degrees  C.  or 
after  three  to  eight  hours  at  26  to  28  degrees  C.  and  then 
brought  into  a  normal  temperature)  continue  to  develop  at  a 
slower  rate  than  eggs  under  normal  conditions.  The  yolk- 
hemisphere  of  the  Qgg  is  first  affected,  so  that  the  cleavage- 
furrows  do  not  appear  in  it.  The  injured  or  dead  half  of  the 
Q^g  lies  below,  and  the  segmented  portion  above. 

Hertwig  obtained  similar  results  by  cooling  the  eggs.  Soon 
after  fertilization  the  eggs  were  placed  in  water  at  zero  C.  and 
kept  there  for  twenty-four  hours.  During  that  time  they  did 
not  segment,  but  when  brought  back  to  a  higher  (normal) 
temperature,  the  Qgg  divided  into  two,  four,  etc.,  blastomeres; 

168 


Ch.  XVI]     EFFECTS  OF  TEMPERATURE  AND  LIGHT  169 

nevertheless,  as  subsequenf  development  showed,  the  egg  had 
been  injured.  Many  of  these  eggs  developed  in  the  same  way 
as  did  those  kept  at  a  temperature  of  25  degrees  C,  i.e.  the 
segmentation  of  the  yolk-hemisphere  was  retarded. 

Schultze  ('95)  has  also  made  some  experiments  on  the 
eggs  of  Rana  fusca  in  which  the  eggs  were  subjected  to  a 
temperature  of  zero  C.  Embryos  in  the  following  stages  of 
development  were  used  :  stage  A,  when  the  dorsal  lip  of  the 
blastopore  had  just  appeared ;  stage  B,  at  the  end  of  the  "gas- 
trula "  period ;  stage  C,  embryos  with  closed  medullary  folds. 
Three  days  after  these  had  been  placed  in  a  chamber  at  zero  C. 
they  were  examined  and  found  in  the  same  stage  as  when  put 
into  the  cold.  Some  of  the  eggs  were  then  removed,  and  con- 
tinued to  develop  normally  at  a  higher  temperature.  After 
fourteen  days  in  the  cold  the  remaining  eggs  were  examined. 
The  eggs  were  still  in  the  same  stage  as  when  put  into  the 
cold  chamber,  but  those  of  stage  C  had  died.  The  others 
developed  normally  when  brought  into  a  liigher  temperature. 

Thus  while  Hertwig  found  that  the  eggs  of  Rana  fusca  were 
injured  by  only  twenty-four  hours  at  a  temperature  of  zero  C, 
Schultze  saw  that  certaiii  stages^  at  least,  were  not  affected  by 
fourteen  days  at  the  same  temperature.  It  is  to  be  noted  that 
Hertwig  put  the  eggs  into  cold  water  soon  after  fertilization, 
while  Schultze  used  later  stages  of  development. 

Not  only  is  the  rate  of  development  of  the  frog-embryo 
affected  by  the  temperature,  but  also  by  the  kind  of  light  in 
which  it  develops.  Schnetzler  in  1874  compared  the  develop- 
ment of  Rana  temporaria  in  white  and  in  green  light.  The  con- 
ditions of  the  two  sets  of  embryos  were  nearly  the  same  except 
as  regards  the  kind  of  light.  The  embryos  developed  much 
faster  in  the  white  light,  and  the  tadpoles  underwent  sooner 
their  metamorphoses.  Yung  ('78)  made  a  much  more  careful 
and  elaborate  series  of  experiments  in  which  the  eggs  and 
embryos  were  subjected  to  a  series  of  different  lights.  Instead 
of  colored  glass,  which  is  seldom  monochromatic,  Yung  used 
solutions  of  different  sorts.  The  eggs  were  placed  in  a  vessel 
containing  about  5  litres  of  water ;  this  vessel  was  then  placed 
in  a  larger  vessel  of  the  same  form.  A  space  of  5  to  10  mm. 
was  left  between  the  two  vessels.     This  space  was  filled  with 


170  DEVELOPMENT  OF   THE   FROG'S  EGG        [Ch.  XYI 

a  fluid  that  allows  only  certain  parts  of  the  spectrum  to 
pass  through.  The  top  of  the  dish  containing  the  eggs  was 
covered  by  an  opaque  lid.  An  alcoholic  solution  of  "  fuchsine 
cerise "  was  used  to  produce  a  monochromatic  red  light ;  a 
solution  of  potassium  chromate  for  a  yellow  light  (although 
this  allows  a  little  red  and  green  to  pass  through)  ;  a  solution 
of  nitrate  of  nickel  (which  is  perfectly  monochromatic)  for  a 
green  light;  an  alcoholic  solution  of  aniline  "bleu  de  Lyon" 
for  a  blue  light ;  and  an  alcoholic  solution  of  aniline  violet  for 
a  violet  light.  Parallel  experiments  took  place  in  the  daylight 
("  white  light ")  and  in  the  dark.  The  other  conditions  were 
the  same  for  all  the  aquaria ;  they  had  the  same  amount  of 
water,  the  same  extent  of  surface  for  aeration,  the  same  tempera- 
ture^ and  were  placed  in  the  same  position  before  a  window. 
Eggs  of  R.  esculenta  and  of  R.  temporaria  were  used. 

At  the  end  of  seven  days  it  could  be  seen  that  the  embryos 
in  the  violet  and  in  the  blue  light  were  more  vigorous  and  in  a 
later  stage  of  development  than  the  others.  At  the  same  time 
the  development  in  the  red  and  in  the  green  was  retarded.  At 
the  end  of  a  month  the  tadpoles  were  in  good  condition,  and  the 
following  table  shows  their  mean  length  in  each  aquarium. 

Larv^  of  Rana  Esculenta  at  the  End  of  One  Month. 


Violet. 

Blue. 

Yellow. 

White. 

Dark. 

Red. 

Green, 

27 

24 

22.8 

23 

19.6 

19.1 

15.1 

The  breadth  of  the  embryos  shows  the  same  differences.  It 
is  interesting  to  see  that  in  the  red  and  in  the  green  light  the 
tadpoles  were  even  less  developed  than  those  in  the  white  light 
or  even  in  the  dark.  The  result  of  this  series  of  experiments 
on  R.  esculenta  agrees  with  other  experiments  made  by  Yung 
at  different  times,  upon  other  species  of  frogs  and  upon  other 
animals. 


APPENDIX 

METHODS   OF  PRESERVATION,  ETC. 

For  general  purposes  the  eggs  and  embryos  may  be  pre- 
served in  a  saturated  solution  of  picric  acid  in  seventy  per  cent, 
alcohol  to  which  a  little  sulphuric  acid  has  been  added  (as 
in  Kleinenberg's  picro-sulphuric  solution).  The  segmenting 
eggs  or  the  early  stages  of  the  embryo  surrounded  by  the 
jelly  should  be  put  directly  into  the  fluid.  Each  egg  should 
have,  however,  the  outer  jelly-coats  cut  off  with  a  pair  of 
scissors,  and  it  is  well  to  use  an  abundance  of  the  preserv- 
ing solution.  Older  embryos  may  be  shelled  out  in  the  pre- 
serving fluid  with  sharp  needles.  After  from  three  to  five 
hours  the  eggs  or  embryos  are  transferred  to  seventy  per 
cent,  alcohol,  which  is  changed  several  times ;  they  should 
be  kept  for  several  days  in  eighty  per  cent,  alcohol.  In 
this  alcohol  (eighty  per  cent.)  the  inner  egg-membrane  slowly 
separates  from  the  Qgg^  and  can  be  easily  removed,  after  which 
the  Qgg  is  preserved  permanently  in  eighty-five  per  cent,  to 
ninety  per  cent,  alcohol.  Corrosive-acetic  solution  gives  good 
results  with  older  embryos.  For  the  early  stages  of  fertiliza- 
tion and  of  extrusion  of  the  polar  bodies  the  following  solution 
is  to  be  recommended  :  one  per  cent,  chromic  acid,  twenty-five 
parts  ;  water,  seventy  parts ;  glacial  acetic  acid,  five  parts. 
Boiling  water  also  gives  good  results. 

Difiiculty  is  often  found  in  cutting  the  eggs  on  account  of 
the  brittleness  of  the  yolk-portion ;  but  if  the  following  method 
is  carefully  followed,  there  will  be  no  trouble  in  this  regard. 
The  preserved  Qgg  or  embryo  is  put  into  absolute  alcohol  from 
two  to  five  hours,  turpentine  two  to  three  hours,  soft  paraf- 
fine  a  half-hour  (change  once),  hard  paraffine  a  half-hour. 
The  melting-point  of   the   hard    parafiine   should  be  from  56 

171 


172  DEVELOPMENT   OF  THE  FROG'S  EGG 

to  58  degrees  C.  The  egg  miist  then  be  cut  at  a  temperature 
of  seventy-five  to  eighty  degrees  Fahrenheit  (24  to  26  degrees 
C);  one  often  succeeds  best  if  the  microtome  is  placed  in  the 
sunlight  during  the  cutting. 

The  segmentation-stages  do  not  need  to  be  stained.  The 
okler  embryos  stain  well  in  toto  in  borax  carmine  or  in 
hsematoxylin  on  the  slide.  Fresh  material  cuts  and  stains 
better  than  that  long  preserved. 

Formalin  preserves  eggs  and  jelly  most  admirably  for  dem- 
onstration. The  segmentation-stages  show  particularly  well 
when  preserved  (permanently)  in  this  solution. 


LITERATURE 

Assheton,  R. 

'94,  a.     On  the  Phenomenon  of  the  Fusion  of  the  Epiblastic  Layers  in 

the  Rabbit  and  in  the  Frog.  Quart.  Jour.  Micr.  Science,  XXXVII,  '94. 
'94,  b.     On  the  Growth  in  Length  of  the  Frog  Embryo.     Quart.  Jour. 

Micr.  Science,  XXXVII,  '94. 
'96.     Notes  on  the  Ciliation  of  the  Ectoderm  of  the  Amphibian  Embryo. 

Quart.  Jour.  Micr;  Science,  XXXVIII,  '96. 
Von  Baer,  K.  E. 

'28.     Ueber  Entwickelungsgeschichte  der  Thiere,  '28  and  '37. 

'28.     Geschichte    des    Froschembryo.      Burdach,   Die    Physiologie    als 

Erfahrungswissenschaft,  II,  '28. 
'34.     Die  Metamorphose  des  Eies  der  Batrachier  von  der  Erscheinung 

des  Embryo  und  Folgerung  aus  ihr  fUr  die  Theorie  der  Erzeugung. 

Miiller's  Archiv,  '34. 
Van  Bambeke,  Ch. 

'68.     Recherches   sur  le   developpement  du  Pelobate   brun.   Memoires 

couronnes  de  I'Acad.  Roy.  des  Sc.  de  Belgique,  XXXIV,  '68. 
'70.     Sur   les  trous  vitellms  que    presentent    les    oeufs    fecondes    des 

amphibiens.  Bull,  de  I'Acad.  Roy.  d.  Sc.  de  Belgique,  (II),  XXX,  '70. 
'80,  a.  Fractionnement  de  I'oeuf  des  Batraciens.  Arch,  de  Biologic,  I,  '80. 
'80,  b.     Xouvelles  recherches  sur  I'embryologie  des  Batraciens.     Arch. 

de  Biologie,  I,  '80. 
'80,  c.    Formation  des  feuilletsembryonnairesetdelanotochordechezles 

Urodeles.     Bull,  de  I'Acad.  Roy.  des  Sc.  de  Belgique,  (II),  XLIX,  '80. 
Barfurth,  D. 

'93,  a.     Halbbildung  oder  Ganzbildung  von  halber  Grosse.     Anat.  Anz., 

VIII,  '93. 
'93,  b.     Experimentelle    Untersuchungen    iiber    die    Regeneration    der 

Keimblatter  bei  den  Amphibien.     Anat.  Hefte,  III,  '93. 
'93,  c.     Ueber  organbildende  Keimbezirke  und  kiinstliche  Missbildungen 

des  Amphibieneies.     Anat.  Hefte,  III,  '93. 
Beddard,  F.  E. 

'94.    Notes  upon  the  Tadpole  of  Xenopus  Isevis  (Dactylethra  capensis). 

Proc.  Zool.  Soc,  London,  '94. 

173 


174  DEVELOPMENT   OF   THE   FROG'S  EGG 

Bellonci,  G. 

'86.     Sui  nuclei  palimorfii  delle  cellule  sessuali  degli  aiifibi,  '86. 
Benecke,  B. 

'80.     Ueber  die  Entwickelung  des  Erdsalamanders.     Zool.  Anz.,  Ill,  '80. 
Bergmann. 

'41.     Die   Zerkluftung  und    Zellenbildung  im   Froschdotter.     Miiller's 

Archiv,  '41. 
Bernard  und  Bratuschek. 

'91.     Der  Nutzen  der  Schleimhiillen  fiir  die  Froscheier.     Biol.  Centralb., 

XI,  '91. 
Bertacchini,  P. 

'89.     Sui  fenomeni  di  divisione  delle  cellule  seminali  primitive  nella 

Rana  temporaria.     Rassegna  Sc.  Med.,  IV,  '89. 
Born,  G. 

'81.     Experimentelle   Untersuchungen   iiber    die    Entstehung  der   Ge- 

schlechtsunterschiede.     Breslauer  arztl.  Zeitschr.,  No.  3,  '81. 
'82.     Ueber  Doppelbilduiigen  beim  Froschund  deren  Entstehung.   Bres- 
lauer arztl.  Zeitschr.,  No.  14,  '82. 
'83,  a.     Biologische  Untersuchungen,  T.    Ueber  den  Einfluss  der  Scliwere 

auf  das  Froschei.     Pfliiger's  Archiv,  XXXII,  '83. 
'83,  b.     Beitrage  zur  Bastardirung  zwischen  den  einheimischen  Anuren- 

arten.     Pfliiger's  Archiv,  XXXII,  '83. 
'84,  a.     Ueber  die  inneren  Vorgange  bei  der  Bastardbefruchtung   der 

Froscheier.     Breslauer  arztl.  Zeitschr.,  No.  16,  '84. 
'84,  b.     Ueber  den    Einfluss    der   Schwere    auf   das    Froschei.     Verh. 

d.    Med.    Section    d.    Schles.    Ges.    f.    vaterl.    Cultur.       April    4, 

'84. 
'85.     Biologische   Untersuchungen  iiber  den  Einfluss  der  Schwere  auf 

das  Froschei.     Archiv  f.  Mikr.  Anat.,  XXIV,  '85. 
'86.     Weitere   Beitrage   zur  Bastardirung  zwischen  den  einheimischen 

Anuren.     Archiv  f .  Mikr.  Anat.,  XXVII,  '86. 
'87.     Ueber  die  Furchung  des  Eies  bei  Doppelbildung.     Breslauer  arztl. 

Zeitschr.,  No.  15,  '87. 
'92.     Die  Reifung  des  Amphibieneies  und  die  Befruchtung  unreifer  Eier 

bei  Triton  taeniatus.     Anat.  Anz.,  VII,  '92. 
'93.     Ueber  Druckversuche  an  Froscheiern.     Anat.  Anz.,  VIII,  '93. 
'94,  a.     Die  kiinstliche  Vereinigung  lebender  TheilstUcke  von  Amphibien- 

Larven.     Jahresb.  d.  Schles.  Ges.  f.  vaterl.  Cultur.    Med.    Section. 

Juni  8,  '94. 
'94,  b.     Die  Structur  des  Keimblaschens  im  Ovarialei  von  Triton  tsenia- 

tus.     Archiv  f.  Mikr.  Anat.,  XLIII,  '94. 
'94,  c.     Neue  Compressionsversuche  an  Froscheiern.     Jahresb.  d.  Schles. 

Ges.  f.  vaterl.  Cultur.     Zool.  Bot.  Section.     10  Mai,  '94. 
Cramer,  H. 

'48.     Bemerkungen    liber    das    Zellenleben   in   der  Entwickelung  des 

Froscheies.     Miiller's  Archiv,  '48. 


LITERATURE  175 

Cucati,  G. 

'90.     Spermatogenesi  nella  Rana  esculenta.     Anat.  Anz.,  V,  '90. 
Driesch,  H. 

'95.     Zur  Analysis  der  Potenzen  embryonaler  Organzellen.     Archiv  f. 

Entwickelimgsmechanik  der  Organismeii,  II,  '95. 
'96,  a.     Betrachtuiigen  iiber  die  Organisation  des  Eies  und  ihre  Genese. 

Archiv  f.  Entwickelungsmechanik  der  Orgauisnien,  IV,  '96. 
'96,  b.     Die    Maschinentheorie    des    Lebens.       Biol.    Centralb.,    XVI, 
'96. 
Durham. 

'86.     Note  on  the  Presence  of  a  Neurenteric  Canal  in  Rana.      Quart. 
Jour.  Micr.  Science,  XXVI,  '86. 
Ecker,  A. 

'51.     Icones  physiologicae,  '51-'59. 
Endres,  H. 

'94.     Ueber  Anstichversuche  an  Froscheiern.    Jahresb.  d.  Schles.  Ges.  f. 

vaterl.  Cultur.     Zool.  Bot.  Section.     Xov.  15,  '94. 
'95.     Ueber  Anstich-  und  Schnurversuche  an  Eiern  von  Triton  tseniatus. 
Jahresb.  d.  Schles.  Ges.  f.  vaterl.  Cultur,  '95. 
Endres,  H.,  und  Walter,  H.  E. 

'95.     Anstichversuche  an  Eiern  von   Rana  fusca.     Archiv  f.  Entwick- 
elungsmechanik d.  Organismen,  II,  '95. 
Von  Erlanger,  R. 

'91,  a.     Zur  Blastoporusfrage  bei  den  anuren  Amphibien.     Anat.  Anz., 

VI,  '91. 
*91,  b.     Ueber  den  Blastoporus  der  anuren  Amphibien,  sein  Schicksal 
und  seine  Beziehungen  zum  bleibenden  After.     Zool.  JahrbUcher. 
Abt.  f.  Anat.  und  Ontog.,  IV,  '91. 
Eycleshymer,  A.  C. 

'92.     Paraphysis   and    Epiphysis   in   Amblystoma.      Anat.   Anz.,   VII, 

'92. 
'93.     The  Development  of  the  Optic  Vesicles  in  Amphibia.    Jour.  ]\Iorph., 
VIII,  '93. 
Fick,  R. 

'93.     Ueber  die  Reifung  und  Befruchtung  des  Axolotleies.     Zeitschr.  f. 
wiss.  Zool.,  LVI,  '93. 
Field,  H.  H. 

'91.     The   Development  of    the   Pronephros   and   Segmental    Duct   in 

Amphibia.     Bull.  Museum  of  Comp.  Zool.,  XXI,  '91. 
'93.     Ueber  die  Gefassversorgung  und  die  allgemeine  Morphologie  des 

Glomus.     Anat.  Anz.,  VIII,  '93. 
*94.     Die  Vomierenkapsel,  ventrale  Musculatur  und   Extremifatenan- 

lagen  bei  den  Amphibien.     Anat.  Anz.,  IX,  '94. 
'95.     Bemerkungen   iiber   die    Entwickelung   der  Wirbelsaule  bei   den 
Amphibien;    nebst   Schilderung  eines   abnormen  Wirbelsegmentes. 
Morph.  Jahrbuch,  XXII,  '95. 


176  DEVELOPMENT   OF   THE   FROG'S  EGG 

Flemming,  W. 

'87.     Xeiie  Beitrage  zur  Kenntniss  der  Zelle.     Archiv  f.  Mikr.  Anat., 
XXIX,  '87. 
Fiirbringer,  M. 

77.     Zur  Entwickelung  der  Amphibienniere.     Dissertation,  '77. 
'78.     Zur  vergleichenden   Anatomie  und  Entwickeluiigsgeschichte  der 
Excretionsorgane  der  Yertebraten.     Morph.  Jahrbuch,  IV,  '78. 
Gurwitsch,  A. 

'95.  Ueber  die  Einwirkung  des  Lithionchlorids  auf  die  Entwickelung 
der  Frosch-  und  Kroteneier  (R.  fusca  und  Bufo  vulg.).  Anat.  Anz., 
XI,  '95. 
'96.  Ueber  die  formative  Wirkung  des  veran-derten  chemischen  Mediums 
auf  die  embryonale  Entwickelung.  Archiv  f.  Entwickelungsme- 
chanik  d.  Organismen,  III,  '96. 
Gasser,  E. 

'82.     Zur  Entwickelung  von  Alytes  obstetricans.     Sitz-Ber  d.  Naturf .  Ges. 
Marburg,  No.  5,  '82. 
Gebhardt,  W. 

'94.     Ueber  die  Bastardirungvon  Rana  esculenta  mit  Rana  arvalis.     Dis- 
sertation, Breslau,  '94. 
Goette,  A. 

'75.     Die  Entwickelungsgeschichte  der  Unke,  '75. 
Von  Griesheim,  A. 

'82.     Kiinstliche  Befruchtung  der  Eier  von  Rana  fusca.     Dissertation, 
'82. 
Von  Griesheim,  A.,  Kochs,  W.,  Pfliiger,  E. 

'81.     Beitrage  zur  Physiologie  der  Zeugung.  Pfliiger's  Archiv,  XXVI,  '81. 
Herlitzka,  A. 

'95.     Contributo  alio  studio  della  capacita  evolutiva  dei  due  primi  blas- 
tomeri  nell'  novo  di  tritone  (Triton  cristatus).     Archiv  f.  Entwick- 
elungsmechanik  d.  Organismen,  II,  '95. 
H6ron  Royer  et  Ch.  van  Bambeke. 

'81.     Sur  les  Caracteres  fournis  par  la  bouche  des  Tetards  des  Batraciens 
anoures  d'Europe.     Bull.  Soc.  Zool.  d.  France,  VI,  '81. 
Hertwig,  0. 

'77.  .  Beitrage  zur  Kenntniss  der  Bildung,  Befruchtung,  und  Theilung 

des  thierischen  Eies,  II  Theil.      Morph.  Jahrbuch,  III,  '77. 
'82.     Die   Entwickelung  des   mittleren    Keimblattes  der  Wirbelthiere. 

Jena.  Zeitschr.  f .  Xaturw.,  XV  und  XVI,  '82-'83. 
'85,  a.     Das  Problem  der  Befruchtung  und  der  Isotropic  des  Eies,  eine 

Theorie  der  Vererbung.     Jena.  Zeitschr.,  XVIII,  '85.  . 
'85,  b.     Welchen  Einfluss  iibt  die  Schwerkraft  auf  die  Theilungen  der 

Zellen.     Jena.  Zeitschr.,  XVIII,  '85. 
'85,  c.     Ueber   das  Vorkommen  Spindeliger   Korper  im  Dotter  junger 

Froscheier.     Morph.  Jahrbuch,  X,  '85. 
'92.     Urmund  und  Spina  bifida.     Archiv  f.  Mikr.  Anat.,  XXXIX,  '92. 


LITERATURE  177 

Hertwig,  0.  {continued). 

'93,  a.     Experimentelle  Untersuchungen  liber  die  ersten  Theilungen  des 
Froscheies  und  ihre  Beziehungen  zu  der  Organbildung  des  Embryos. 
Sitzungsb.  d.  k.  Preuss.  Akad.  d.  Wiss.  zu  Berlin,  '93. 
'93,  b.     Ueber   den  Werth   der  ersten   Furchungszellen   fUr  die  Organ 

bildung  des  Embryo.     Archiv  f.  Mikr.  Anat.,  XLII,  -93. 
*94.     Ueber  den  Einfluss  ausserer   Bedingungen  auf  die  Entwickelung 
des  Froscheies.     Sitzungsb.  d.  k.  Preuss.  Akad.  d.  Wiss.  zu  Berlin, 
XVII,  '94. 
'95.     Beitrage  zur  experimentellen  Morphologie  und  Entwickelungsge- 
schichte,  No.  1.     Archiv  f.  Mikr.  Anat.,  XLIV,  '95. 
Higgenbotham,  J. 

'50.  Influence  of  Physical  Agents  on  the  Development  of  the  Tad- 
pole of  the  Triton  and  the  Frog.  Phil.  Trans.  Roy.  Soc,  London, 
'50. 
*63.  Influence  des  agents  physiques  sur  le  developpement  du  tetard  de 
la  grenouille.  Jour,  de  la  Physiologie  de  I'homme  et  des  animaux. 
VI,  '63. 
Hinckley,  Mary  H. 

'82.     Xotes  on  the  Development  of  Rana  sylvatica.     Proc.  Boston  Soc. 
Nat.  History,  '82. 
His,  W. 

'74.     Unsere  Korperform,  '74. 
Hochstetter,  F. 

'94.     Entwickelung  des  Venensystemes  der  Wirbelthiere.     Ergebnisse 
der  Anatomic  und  Entwickelungsgeschichte,  III,  '94. 
Houssay,  F. 

'90.     fitudes   d'embryologie   sur  les  vertebres.     L'Axolotl.     Archiv  de 
Zool.  exper.  et  gen.,  (ID),  VIII,  '90. 
De  ITsle,  A. 

*73.     De  I'hybridation  chez  les  amphibies.      Ann.  des  sc.  naturelles, 
XVII,  '73. 
Johnson,  A. 

'84.     On  the  Fate  of  the  Blastopore  and  the  Presence  of  a  Primitive 
Streak  in  the  Newt  (Triton  cristatus).     Quart.  Jour.  Micr.  Science, 
XXIV,  '84. 
Johnson,  A.,  and  Sheldon,  L. 

'86.     Notes  on   the   Development  of  the  Newt.     Quart.   Jour.   Micr. 
Science,  XXVI,  '86. 
Jordan,  E.  0.,  and  Eycleshymer,  A. 

'92.     The  Cleavage  of  the  Amphibian  Ovum.     Anat.  Anz.,  VII,  '92. 
Jordan,  E.  0. 

'93.     The  Habits  and  Development  of  the  Newt.    Jour,  of  Morph.,  VIII, 
'93. 
Jordan,  E.  0.,  and  Eycleshymer,  A.  C. 

'94.     On  the  Cleavage  of  Amphibian  Ova.     Jour.  Morph.,  IX,  '94. 


178        DEVELOPMENT  OF  THE  FROG'S  EGG 

Kolessnikow,  N. 

78.     Ueber  die  Eientwickelung  bei  Batrachiern  und  Knochenfischen. 
Archiv  f.  Mikr.  Anat.,  XV,  78. 
Kupffer,  C. 

'82.     Ueber  aktive  Betheiligung  des  Dotters  am  Befruchtungsakte  bei 
Bufo  variabilis  und  vulgaris.    Sitzungsb.  d.  math.-physik  Classe  d.  k. 
b.  Akad.  d.  Wiss.  zu  Miinchen,  XII,  '82. 
Lataste,  F. 

'78.     Tentatives  d'hybridation  chez  les  Batraciens  anoures  et  urodeles. 
Bulletin  de  la  Societe  zoologique  de  France,  III,  '78. 
Lwoff,  B. 

'94.     Die   bildung  der  primaren   Keimblatter   und  die  Enstehung  der 
Chorda  und  des  Mesoderms  bei  den  Wirbelthieren.     Bull,  de  la  Soc. 
imper.  des  Xaturalistes  de  Moscou,  VIII,  '94. 
Marshall,  A.  M.,  and  Bias,  E.  J. 

'90.     The  Development  of  the  Kidneys  and  Fat  Bodies  in  the  Frog. 
Stud.  Biolog.  Lab.  Owens  College,  II,  '90. 
Marshall,  A.  M. 

'90.     The  Development  of  the  Blood  Vessels  in  the  Frog.     Stud.  Biolog. 

Lab.  Owens  College,  II,  '90. 
'93.     Vertebrate  Embryology,  '93. 
McDonnell,  B. 

'57.     Expose  de  quelques  experiences  concernant  I'influence  des  agents 
physiques   sur  le   developpement  du  tetard   de   la   grenouille  com- 
mune.    Jour,  de  la  Physiologie  de  I'homme  et  des  animaux.     (I),  II, 
'57. 
Massart,  J. 

'89.     Sur  la  penetration  des  spermatoides  dans  I'oeuf  de  la  grenouille. 
Bull,  de  I'Acad.  Roy.  des  Sci.  de  Belgique,  (III),  XVIII,  '89. 
Maurer,  F. 

'88.     Die  Kiemen  und  ihre  Gefasse  bei  Anuren  und  Urodelen  Amphibien. 
Morph.  Jahrbuch,  XIV,  '88. 
Meves,  F. 

'91.     Ueber  amitotische  Kerntheilung  in  den  Spermatogonien  des  Sala- 
manders.    Anat.  Anz.,  VI,  '91. 
'96.     Ueber   die   Entwickelung   der  mannlichen   Geschlechtszellen  von 
Salamandra  maculosa.     Archiv.  f.  Mikr.  Anat.,  XL VIII,  '96. 
Moquin-Tandon,  G. 

'76.     Recherche  sur  les  Premiere  Phases  du  Developpement  des  Batra- 
ciens anoures.     Ann.  des  sciences  naturelles,  (VI),  III,  '76. 
Morgan,  T.  H. 

'89.     On  the  Amphibian  Blastopore.     Studies  from  the  Biol.  Lab.  Johns 

Hopkins  Univ.,  IV,  '89. 
'91.     Some  Notes  on  the  Breeding  Habits  and  Embryology  of  Frogs. 

American  Naturalist,  XXV,  '91. 
'94.     The  Formation  of  the  Embryo  of  the  Frog.     Anat.  Anz.,  IX,  '94. 


LITERATURE  179 

Morgan,  T.  H.  {continued). 

'95.     Half-Embryos  and  Whole-Embryos  from  one  of  the  first  two  Blas- 
tomeres  of  the  Frog's  Egg.     Anat.  Anz.,  X,  '95. 
Morgan,  T.  H.,  and  Tsuda  Um6. 

'93.     The  Orientation  of  the  Frog's  Egg.     Quart.  Jour.  Micr.  Science, 
XXXV,  '93. 
Newport,  G. 

'51.     On  the  Impregnation  of  the  Ovum  in  the  Amphibia.     Phil.  Trans. 
Roy.  Soc,  London,  '51. 
Nussbaum. 

'93.     Zur  Entwickelungsgeschichte  der  embryonalen  Gefassendothelien 
und  der  Blutkorperchen  bei  den  Anuren  (Rana  temporaria).     Abh. 
Akad.  der  Wiss.  in  Krakau,  XXII.    (Reprinted  in  Biol.  Centralblatt, 
XIII,  '93.) 
Nussbaum,  M. 

'95.     Zur  Mechanik  der  Eiablage  bei  Kana f usca.     Archiv  f .  Mikr.  Anat., 
XLYI,  '95. 
Orr,  H. 

'88.     Note  on  the  Development  of  Amphibians.     Quart.  Joui\  Micr. 
Science,  XXIX,  '88. 
Von  Per6nyi,  J. 

'89.     Die    Entwickelung    der    Keimblatter  und   der   Chorda  in  neuer 
Beleuchtung.     Anat.  Anz.,  IV,  '89.     (Page  587.) 
Pfliiger,  E. 

'82.  I.  Hat  die  Concentration  des  Saraens  einen  Einfluss  auf  das 
Geschlecht?  II.  Ueber  die  das  Geschlecht  bestimmenden  Ursachen 
und  die  Geschlechtverhaltnisse  der  Frosche.  III.  Ueber  die  partheno- 
genetische  Furchung  der  Eier  der  Amphibien.  IV.  Wirkt  der  Saft 
nicht  brunstiger  Mannchen  befruchtend?  V.  Die  Bastardzeugung 
bei  den  Batrachien.  VI.  Versuche  der  Befruchtung  iiberreifer  Eier. 
VII.  Zur  Entwickelungsgeschichte  der  GeburtsheKerkrdte  (Alytes 
obstetricans).  Pfltiger's  Archiv,  XXIX,  '82. 
'83.     Ueber  den  Einfluss  der  Schwerkraft  auf  die  Theilung  der  Zellen. 

I,  II,  III,  Theil.    Pfluger's  Archiv,  XXXI,  XXXII,  '83. 
'84.     Ueber  die  Einwu'kung  der  Schwerkraft  und  anderer  Bedingungen 
auf    die   Richtung   der    Zelltheilung.      Pfltiger's  Archiv,  XXXIV, 
'84. 
Pfliiger,  E.,  und  Smith,  W.  J. 

'83.     Untersuchungen  iiber  Bastardirung  der  anuren  Batrachier  und  die 
Principien  der  Zeugung.     Pfltiger's  Archiv,  XXXII,  '83. 
Ploetz,  A.  J. 

'90.     Die  Vorgange  in  den  Froschhoden  unter  dem  Einfluss  der  Jahres- 
zeit.     Archiv  f.  Anat.  u.  Phys.,  (Supplement-Band)  '90. 
Prevost  et  Dumas. 

'24.     Troisieme  Memoire.     De  la  generation  dans  les  Mammifers.    Ann. 
des  sc.  naturelles,  II,  '24. 


180  DEVELOPMENT   OF   THE   FROG'S   EGG 

Rabl,  C. 

'86.     Ueber  die  Bildung  des  Herzens  der  Amphibien.    Morph.  Jahrbuch, 
XII,  '86. 
Vom  Rath,  0. 

'93.     Beitrage  zur  Kenntniss  der  Spermatogenese  von  Salamander  macu- 
losa.    Zeitschr.  f.  wiss.  Zool.,  LVII,  '93. 
Rauber,  A. 

'82.     Xeue  Grundlegungen  zur  Kenntnis  der  Zelle.    Morph.  Jahrbuch, 

VIII,  '82. 

'83.     Furchung  und  Achsenbildung  der  Wiebelthiere.     Zool.  Anz.,  VI, 

'83. 
'86,  a.     Personaltheil  und  Germinaltheil  des  Individuum.     Zool.  Anz., 

IX,  '86. 

'86,  b.     Furchung  mid  Achsenbildung.     II.     Zool.  Anz.,  IX,  '86. 
Reichert,  K.  B. 

'41.  Ueber  den  Furchungsprocess  der  Batrachier-Eier.  Midler's  Archiv. 
'41. 

'46.  Der  Furchungsprocess  und  die  sogenannte  Zellenbildung  um  In- 
haltsportionen.     Miiller's  Archiv,  '46. 

'61.     Der  Faltenkranz  an  den  beiden  ersten  Furchungskugeln  des  Frosch- 
dotters  und  seine  Bedeutung  fiir  die  Lehre  von  der  Zelle.     Midler's 
Archiv,  '61. 
Remak,  R. 

'55.     Untersuchung  iiber  die  Entwickelung  der  Wirbelthiere,  '55. 
Robinson,  A.,  and  Assheton,  R. 

'91.     The  Formation  and  Fate  of  the  Primitive  Streak,  with  Observa- 
tions on  the  Archenteron  and  Germinal  Layers  of  Rana  temporaria. 
Quart.  Jour.  Micr.  Science,  XXXII,  '91. 
Rossi,  U. 

'90.     Contributo   alia  maturazione   delle  nova   degli    Amfibii.      Anat. 
Anz.,  V,  '90. 
Roux,  W. 

'83.  Ueber  die  Zeit  der  Bestimmung  der  Hauptrichtungen  des  Frosch- 
embryo,  '83. 

'84-'91.     Beitrage  zur  Entwickelungsmechanik  des  Embryo. 

No.  I.  Zur  Orientirung  iiber  einige  Probleme  der  embryonalen  Ent- 
wickelung.    Zeitschr.  f.  Biologic,  XXI,  '85. 

No.  II.  Ueber  die  Entwickelung  der  Froscheier  bei  Aufhebung 
der  richtenden  Wirkung  der  Schwere.  Breslauer  arztl.  Zeitschr., 
'84. 

No.  III.  Ueber  die  Bestimmung  der  Hauptrichtungen  des  Frosch- 
enibryo  im  Ei  und  iiber  die  erste  Theilung  des  Froscheies.  Bres- 
lauer arztl.  Zeitschr.,  '85. 

No.  IV.  Die  Bestimmung  der  Medianebene  des  Froschembryo  durch 
die  Copulationsrichtung  des  Eikernesund  des  Spermakernes.  Archiv 
f.  Mikr.  Anat.,  XXIX,  '87. 


LITERATURE  181 

Ronz,  W.  (continued). 

No.  V.     Ueber  die  kiinstliche  Hervorbringiing  halber  Embryonen  durch 

Zerstbrung  einer  der   beiden  ersten   Furchungskugeln   sowie  iiber 

die  Nachentwickelung  (Postgeneration)  der  fehlenden  Korperhalfte. 

Yirchow's  Archiv,  CXIV,  '88. 
No.  VI.     Ueber  die  moiphologische  Polarisation  von  Eiem  und  Embry- 
onen durch  den  electrischen  Strom.     Sitzungsb.  d.  k.  Akad.  Wiss. 

in  Wien,  CI,  '91. 
'88,  a.     Ueber  die  Lagerung  des  Materials  des  MeduUarrohres  im  gefurch- 

ten   Froschei.     Yerhandlungen  d.  Anat.  Gesell.  zu  Wiirzburg,  '88; 

also  Anat.  Anz.,  Ill,  '88. 
'88,  b.     Zur  Frage  der  Axenbestimmung  des  Embryo  im  Froschei.     Biol. 

Centralb.,  YIII,  '88. 
'89,  a.     Die  Entwickelungsmechanik  der  Organismen,  eine  anatomische 

Wissenschaft  der  Zukunft.     Festrede,  '89. 
'89,  b.     Ueber  die  Entwickelung  der  Extraovates  der  Froscheier.   Jahresb. 

d.  Schles.  Ges.  f.  vaterl.  Cultur,  '89. 
'92,  a.     Ziele  und  Wege  der  Entwickelungsmechanik.     Merkel-Bon net's 

Ergebnisse  der  Anatomie  und  Entwickelungsgeschichte,  II,  '92. 
'92,  b.     Ueber  das  entwickelungsmechanische  Yermogen  jeder  der  beiden 

ersten  Furchungszellen  des  Eies.     Yerhandl.  d.  Anat.  Gesellschaft 

Wien,  '92. 
'93,  a.     Ueber    Mosaikarbeit     und     neuere     Entwickelungshypothesen. 

Anat.  Hefte,  ^lerkel  und  Bonnet,  '93. 
'93,  b.     Ueber  die  Spezifikation  der  Furchungszellen  und  iiber  die  bei 

der  Postgeneration   und    Regeneration    anzunehmenden   Yorgange. 

Biol.  Centralb.,  XIII,  '93. 
'93,  c.     Ueber  die   ersten   Theilungen  des  Froscheies  und  ihre   Bezie- 

hungen  zu  der  Organbildung  des  Embiyo.     Anat.  Anz.,  \T!1I,  '93. 
'93,  d.     Ueber  die   Selbstordnung  der   Furchungszellen.     Berichte   des 

natursv.-med.  Yereins  zu  Innsbruck,  XXI,  '93. 
'94,  a.    Die  Methoden  zur  Hervorbringung  halber  Froschembryonen  und 

zum  Xachweis  der  Beziehung  der  ersten  Furchungsebenen  des  Frosch- 
eies zur  Medianebene  des  Embryo.     Anat.  Anz.,  IX,  '91. 
94,  b.    Ueber  den  Cytotropismus  der  Furchungszellen  des  Grasfrosches 

(Rana  fusca).     Archiv  f.  Entwickelungsmechanik  der  Organismen, 

I,  '94. 
'95.     Gesammelte     Abhandlungen     Uber    Entwickelungsmechanik   der 

Organismen,  '95. 
'96,  a.     Ueber  die  Selbstordnung  (Cytotaxis)  sich  "  beriihrender "  Fur- 
chungszellen des  Froscheies  durch  Zellenzusammenfugung,  Zellentren- 

nung    und    Zellengleiten.      Archiv  f.    Entwickelungsmechanik   der 

Organismen,  III,  '96. 
'96,  b.     Ueber  die  Bedeutung  "geringer  "  Yerschiedenheiten  der  relativen 

Grosse  der  Furchungszellen  fUr  den  Charakter  des  Furchungsschemas. 

Archiv  f .  Entwickelungsmechanik  der  Organismen,  lY,  '96. 


182  DEVELOPMENT  OF  THE  FROG'S  EGG 

Ruckert,  J.,  und  MoUier,  W. 

'89.     Resultate  iiber  die  Entstehung  des  Vornierensystem  bei  Triton, 

Rana  und  Bufo.     Sitz.  Ber.  d.  Ges.  fur  Morph.  u.  Phys.  in  Munchen, 

XIX,  '89. 
Rusconi,  M. 

'26.     Developpement  de  la  grenouille  commune,  '26. 

'36.     Zweiter  Brief  an  E.  H.  Weber.     MuUer's  Archiv,  '36. 

'40.     Ueber  kiinstliche  Befruchtung  von  Fischen  und  iiber  einige  neue 

Versuche  in  Betreff  ktinstlicher  Befruchtung  an  Froschen.     Miiller's 

Archiv,  '40. 
'54.     Histoire  naturelle,   developpement  et  metamorphose  de  la  Sala- 

mandre  terrestre,  '54. 
Samassa,  P. 

'95.     Studien  iiber  den  Einfluss  des  Dotters  auf  die  Gastrulation  und  die 

Bildung  der  primaren  Keirablatter  der  Wirbelthiere.     II.  Amphibien. 

Archiv  f.  Entw.-mechanik  d.  Organismen,  II,  '95. 
Schanz,  F. 

'87.     Das  Schicksal  des  Blastoporus  bei  den  Amphibien.    Jena.  Zeitschr. 

f.  Naturwissenschaft,  XXI,  '87. 
Schmidt,  V. 

'93.     Das    Schwanzende  der  Chorda  dorsalis  bei  den   Wirbelthieren. 

Anat.  Hefte,  II,  '93. 
Schnetzler,  J.  B. 

74.     De  I'influence  de  la  lumiere  sur  le  developpement  des  larves  de 

grenouilles.     Arch,  des  sciences  physiques  et  naturelles,  LI,  '74. 
Schultze,  M. 

'63.     Observationes  nonnullae  de  ovorum  ranarum  segmentatione,  '63. 
Schultze,  0. 

'83.     Beitrage  zur  Entwickelung  der  Batrachien.     Archiv  f.  Mikr.  Anat., 

XXIII,  '83. 
'86.     Ueber  Reifung  und  Befruchtung  des  Amphibieneies.    Anat.  Anz., 

I,  '86. 
'87,  a.     Zur  Entwickelung   des  braunen    Grasfrosches.     Festschrift  f. 

Kolliker,  '87. 
'87,  b.     Die  vitale  Methylenblaureaction  der  Zellgranule.     Anat.  Anz., 

'87. 
'87,  c.     Zurersten  Entwickelung  des  braunen  Grasfrosches  (Ref.  Roux). 

Biol.  Centralb.,  VII,  '87. 
'87,  d.     Ueber  Axenbestimmung  des  Froschembryo.      Biol.  Centralb., 

VII,  '87. 
'87,  e.     Untersuchungen  iiber  die  Reifung  und  Befruchtung  des  Amphibi- 
eneies.    Zeitschr.  f.  wiss.  ZooL,  XLV,  '87. 
'88.     Die  Entwickelung  der  Keimblatter  und  der  Chorda  dorsalis  von 

Rana  fusca.     Zeitschr.  f .  wiss.  ZooL,  XLVII,  '88. 
'89.     Ueber  die  Entwickelung  der  Medullarplatte  des  Froscheies.    Verb- 

d.  phys.  med.  Gesellschaft,  Wurzburg,  XXIII,  '89. 


LITERATURE  183 

Schultze,  0.  (continued). 

'94,  a.     Ueber  die  unbedingte  Abhangigkeit  normaler  thierischer  Gestal- 

tung  von  der  Wirkung  der  Schwerkraft.    Verb.  d.  Anat.  Ges.,  VIII,  '94. 
'94,  b.     Die  ktinstliche  Erzeugung  von  Doppelbildungen  bei  Froschlarven 

mit  Hilfe  abnormer  Gravitationswirkung.     Archiv  f.  Entwickelungs- 

mechanik  der  Organismen,  I,  '94. 
'94,  c.     Ueber  die  Ein wirkung  niederer  Teraperatur  auf  die  Entwickelung 

des  Frosches.     Anat.  Anz.,  X,  '94. 
'94,  d.     Ueber  die  Bedeutung  der  Schwerkraft  fiir  die  organische  Ges- 

taltung  sowie  iiber  die  mit  Hilfe  der  Schwerkraft  mogliche   kiinst- 

liche  Erzeugung  von  Doppehnissbildungen.     Verh.  phys.  med.  Ges. 

zu  WUrzburg,  XXVUI,  '94. 
Schwink,  F. 

*88.     Ueber  die  Gastrula  bei  Amphibieneiern.     Biol.  Centralb.,  VIII, 

'88-'89. 
'89.     Ueber  die  Entwickelung  des  mittleren  Keimblattes  und  der  Chorda 

dorsalis  der  Amphibien.     Miinchen,  '89. 
'90.     Ueber   die   Entwickelung  des   Herzenendothels    der    Amphibien. 

Anat.  Anz.,  V,  '90. 
'91.     Untersuchungen  iiber  die  Entwickelung  des   Endothels  und  der 

Blutkorperchen  bei  Amphibien.     Morph.  Jahrb.,  XVII,  '91. 
Sidebotham,  H. 

'88.     Note  on  the  Fate  of  the  Blastopore  in  Rana  temporaria.     Quart. 

Jour.  Micr.  Science,  XXIX,  '88. 
Spallanzani,  L. 

1785.     Experience  pour  servir  a  I'histoire  de  la  generation,  1785. 
Spencer,  W.  B. 

'85.     On  the  Fate  of  the  Blastopore  in  Rana  temporaria.     Zool.  Anz., 

VIII,  '85. 
'85.     Some  Xotes  on  the  Early  Development  of  Rana  temporaria.     Quart. 

Jour.  Micr.  Science,  XXV,  '85. 
Stahl,  E. 

'88,     Pflanzen  und  Schneckeu.    (Jena,)  '88. 
Strieker,  S. 

'60.     Entwickelungsgeschichte  von  Bufo  cinereus  bis  zum  Erscheinen  der 

ausseren  Kiemen.     Sitzungsb.  d.  k.  Akaderaie  der  Wiss.  zu  Wien, 

XXXIX,  '60. 
'62.     Untersuchungen    iiber    die   ersten   Anlagen   in   Batrachier-Eiern. 

Zeitschr.  f .  wiss.  Zool.,  XI,  '62. 
Swammerdam,  J. 

1737.     Die  Bibel  der  Natur,  1737. 
V.  la  Valette  St.  George. 

'75.     Die  Sperm atogenese  bei  den  Amphibien.     Archiv  f .  Mikr.  Anat., 

XII,  '75. 
'86.     Spermatologische  Beitrage.     Dritte  Mittheilung.     Archiv  f .  Mikr. 

Anat.,  XXVII,  '86. 


184  DEVELOPMENT   OF   THE   FROG'S  EGG 

Vogt,  K. 

'42.     Untersuchungen   iiber   die  Entwickeluugsgeschichte  der  Geburts- 
helferkrbte,  *42. 
Wetzel,  G. 

'95.     Ueber  die  Bedeutung  der  Cirkularen  Furche  in  der  Entwickelung 
der  Schultzeschen  Doppelbildungen  von  Rana  fusca.     Archiv  f.  Mikr. 
Anat.,  XLVI,  '95. 
'96.     Beitrag  zum  Studium  der  kiinstlichen  Doppelmissbildungen  von 
Rana  fusca.     Inaugural  Dissertation.     Berlin,  '96. 
Will,  L. 

'84.     Ueber  die  Entstehung  des  Dotters  und  der  Epithelzellen  bei  den 
Aniphibien  und  Insecten.     Zool.  Anz.,  VII,  '84. 
Wilson,  C.  B. 

'96.     The  Wrinkling  of  Frog's  Eggs  during  Segmentation.     American 
Naturalist,  XXX,  '96. 
Yung,  E. 

'78.     Influence  des  differentes  couleurs  du  spectre  sur  le  developpement 
des  animaux.    Arch,  de  zool.  experimentale  et  generale,  (I,)  VII,  '78, 
and  Arch,  des  sciences  physiques  et  naturelles,  '78. 
'81.     De  I'influence  des  lumieres  colorees  sur  le  developpement  des  ani- 
maux.    Mittheil.  a.  d.  zool.  Station  zu  Neapel,  II,  '81. 
'90.     Propos  scientifiques,  '90. 
Ziegler,  F. 

'92.     Zur  Kenntniss  der  Oberflachenbilder  der  Rana-Embryonen.    Anat. 
Anz.,  VII,  92. 


OTHER   MEMOIRS   REFERRED   TO   IN   TEXT 

Boveri,  Th. 

'89.     Ein  geschlechtlich  erzeugter  Organismus  ohne  mtitterliche  Eigen- 

schaften.     Sitz.   d.    Ges.   f.   Morph.  u.   Physiol,  zu   Munchen,   '89. 

(Translated  in  American  Naturalist,  March,  '93.) 
Chun,  B. 

'92.     Die  Dissogonie  der  Rippenquallen.     Festschrift  f.  Leuckart,  '92. 
Clapp,  C.  M. 

'91.     Some  Points  on  the  Development  of  the  Toad-fish  (Batrachus  Tau) . 

Jour.  Morph.,  V,  '91. 
Driesch,  H. 

'91-93.     Entwickelungsmechanische  Studien. 

No.  I.     Der  Werth  der  beiden  Furchungszellen  der  Echinodermentwick- 

elung.     Zeitschr.  f.  wiss.  Zool.,  LIII,  '91. 
No.  II.     Ueber   die   Beziehungen   des   Lichtes   zur   ersten   Etappe   der 

thierischen  Fornibildung.     Zeitschr.  f.  wiss.  Zool.,  LIII,  '91. 
No.  III.     Die  Verminderung  des  Furchungsmaterials  und  ihre  Folgen. 

Zeitschr.  f.  wiss.  Zool.,  LV,  '92. 


LITERATURE  185 

Driesch,  H.  (continued'). 

Xo.  IV.     Experimentelle  Veranderungen  des  Typus  der  Furchung  und 

ihre  Folgeii.     Zeitschr.  f.  wiss.  ZooL,  LV,  '92. 
No.  V.     Von  der  Furchung  doppeltbefruchteter  Eier.     Zeitschr.  f.  wiss. 

Zool.,  LV,  '92. 
No.  VI.     Ueber  einige  allgemeine  Fragen  der  theoretischen  Morphologie. 

Zeitschr.  f.  wiss.  Zool.,  LV,  '92. 
No.  VIT.     Exogastrula  und  Aneuteria.     Mittheil.  a.  d.  zool.  Station  zu 

Neapel,  XI,  '93. 
No.  VIII.     Ueber  Variation  der  Mikromerenbildung.     Mittheil.  a.  d. 

zool.  Station  zu  Neapel,  XI,  '93. 
No.  IX.     Ueber    die  Vertretbarkeit   der  Anlagen  von  Ektoderm  und 

Endoderm.     Mittheil.  a.  d.  zool.  Station  zu  Neapel,  XI,  '93. 
No.  X.     Ueber  einige  allgemeine  entwickelungsmechanische  Ergebnisse. 

Mittheil.  a.  d.  zool.  Station  zu  Neapel,  XI,  '93. 
'92.     Entwickelungsmechanisches.     Anat.  Anz.,  VII,  '92. 
'93,  a.     Zur   Theorie   der  thierischen    Formbildung.      Biol.    Centralb., 

XIII,  '93. 
'93,  b.     Zur  Verlagerung  der  Blastomeren  des  Echinideneies.      Anat. 

Anz.,  VIII,  '93. 
'94.     Analytische  Theorie  der  Organischen  Entwickelung,  '94. 
Driesch,  H.,  und  Morgan,  T.  H. 

'95.     Zur  Analj^sis  der  ersten  Entwickelungsstadien  des  Ctenophoreneies. 

Archiv  f.  Entwickelungsmechanik  d.  Organismen,  II,  '95. 
Hertwig,  0. 

'90.    Vergleich  der  Ei-  und  Samenbildung  bei  Nematoden.     Archiv  f. 

Mikr.  Anat.,  XXXVI,  '90. 
'94.     Zeit-  und  Streitfragen  der  Biologie,  '94. 
Hertwig,  0.,  und  Hertwig,  R. 

'87.     Ueber  den  Befruchtuugs-  und  Theilungs-vorgang  des  Thierischen 

Eies  unter  dem  Einfluss  ausserer  Agentien.     Jena.  Zeitschr.  Naturw., 

XX,  '87. 
His,  W. 

'94.     Ueber    mechanische    Grundvorgange    thierischer    Formbildung. 

Archiv  f.  Anat.  u.  Phys.,  '94. 
Morgan,  T.  H. 

'93.     Experimental  Studies  on  Teleost  Eggs.     Anat.  Anz.,  VIII,  '93. 
'95.     Studies  of    the   "Partial"   Larvse  of   Sphserechinus.      Archiv  f. 

Entwickelungsmechanik  d.  Organismen,  II,  '95. 
Rabl,  C. 

'89.     Die  Theorie  des  Mesoderms.     Morph.  Jahrbuch,  XV,  '89. 
Vom  Rath,  0. 

'92.     Zur    Kenntniss    der    Spermatogenese    von    Gryllotalpa  vulgaris. 

Archiv  f .  :Mikr.  Anat.,  XL,  '92. 
'95.     Neue  Beitrage  zur  Frage  der  Chromatinreduction  in  der  Samen- 

und  Eireife.     Archiv  f .  Mikr.  Anat.,  XLVI,  '95. 


186  DEVELOPMENT   OF   THE   FROG'S  EGG 

Rauber,  A. 

'80.     Formbildung  und  Formstorung.     IMorph.  Jahrbuch,  VI,  '80. 
Sachs. 

'92.    Die  Anordnung  der  Zellen  in  jiingsten  Pflanzentheilen.     Arbeiten 
d.  botan.  Institute  in  Wurzburg,  II,  '92. 
Schwann,  Th. 

'39.     Mikroskopische  Untersuchungen   Uber   die  Uebereinstimmung  in 
der  Structur  und  Wachsthum  der  Thiere  und  Pflanzen,  '39. 
Weismann,  A. 

'92.     Das  Keimplasma.     Eine  Theorie  der  Vererbung.     '92. 
Whitman,  C.  0. 

'95.     The  Inadequacy  of  the  Cell  Theory  of  Development.     Jour.  Morph., 
VIII,  '93. 
Wilson,  E.  B. 

'92.     The  Cell  Lineage  of  Nereis.     Jour.  Morph.,  VI,  '92. 
'93.    Amphioxus  and  the  Mosaic  Theory.     Jour.  Morph.,  VIII,  '93. 
Zoja,  R. 

'95.  Sullo  svilluppo  dei  blastomeri  isolati  dalle  uova  di  alcune  meduse 
(e  di  altri  organismi).  Archiv  f.  Entwickelungsmechanik  d.  Organ- 
ismen,  I,  '95. 


INDEX 


Adhesive  glands,  62,  165. 
Afferent  branchial  vessels,  153-155. 
Amphioxus,   isolation  of  blastomere, 
127,  131. 

isolation   of  one-fourth    and    one- 
eighth  blastomeres,  133. 
Anus,  60,  62. 

beginning  of,  136-139. 

of  Urodela,  139,  140. 
Aorta,  early  stage,  153. 
Aortic  bulb,  153. 
Archenteron,  66-68,  70. 

of  spina  bifida,  77. 

of  hemiembryo,  108,  109. 

posterior  pocket  of,  136-140. 

enlargement  of,  140-143. 
Ascidians,  half-development,  127. 

isolation  of  blastomere,  131. 
Assheton,  formation  of  archenteron, 
70. 

ciliation  of  embryo,  165. 
Auditory  nerve,  164. 
Auricle  of  heart,  153. 
Axis,  primary,  81. 

secondary,  82. 

tertiary,  82. 

von  Baer,  account  of  cleavage,  48. 

Barock  segmentation,  29. 

Bellonci,  direct  division  of  germ-cells, 

12. 
Bergmann,  account  of  cleavage,  49. 
Bernard  and  Bratuschek,  20. 
Blastomeres,  injury  to,  106-122. 
Blastopore,  50-57. 

overgrowth  of,  50-57,  68. 

injury  to,  79. 

position  of  dorsal  lip,  88. 

in  compressed  egg,  98-101. 

closure  of,  137-140. 

of  Urodela,  139,  140. 
Blastula,  double,  118. 


Blood-corpuscles,  155. 
Body- cavity,  148. 
Bombinator,  pronephros  of,  158. 
Born,  cross-fertilization,  26-28. 

sperm-fluid,  29. 

experiments,  90-92. 

compression  of  egg,  95-99. 

conclusions  from  compressed  egg, 
102. 

cleavage-plane    and    embryo-axis, 
108. 
Boveri,  second  maturation-division,  8, 
9. 

egg-fragments,  30,  31. 
Brain-vesicles,  62. 
Branchial  arches,  145. 
Branchial  vessel,  153,  154. 

development  of,  155. 
Brauer,  second  maturation-division,  8, 

9. 
Bufo,  fertilization  of  egg,  22. 

vulgaris,  cross-fertilization,  26-28. 

communis,  cross-fertilization,  28. 

Cardinal  veins,  153,  157. 

Cellulation  of  yolk,  hemiembryo,  110. 

Centrifugal  force,  effect  of,  92-94. 

Centrifugal  machine,  92-94. 

Cerebellum,  160. 

Cerebral  hemispheres,  160. 

Chabry,  127. 

Chun,  127. 

Cilia,  on  surface,  165. 

Cleavage,  32-41. 

of  compressed  egg,  96-101. 

of  egg  in  centrifugal  machine,  92-94. 
Cleavage-plane,  relation    to   egg-axis, 

82,  85-87. 
Cloaca,  opening  of  segmental  duct  into, 

156. 
Coelom,  148, 150. 

relation  to  pronephros,  156. 


187 


188 


DEVELOPMENT  OF   THE  FROG'S  EGG 


Collecting  tubes  of  pronephros,  156, 

157. 
Communicating  canal,  148. 
Compression  of  egg,  95-105. 
Concrescence,  64,  65,  80. 
Correlation  of  parts,  124-126. 
Cramer,  account  of  cleavage,  49. 
Cranial  nerves,  164. 
Cross-fertilization,  26-30. 
Cross-line,  37,  39. 
Ctenophor,  half-development,  127. 

isolated  blastomere,  129,  130,  132. 

half-larva,  130. 

fragment  of  egg,  131. 

imperfect  embryo,  135. 
Cutaneous  nerves,  164. 
Cuvierians  veins,  153. 

Delamination,  41. 
Development,  direct,  128. 

indirect,  128. 
Differentiation  by  protoplasm,  131. 
Diverticula,  from  aorta,  155. 

from  aortic  bulb,  155. 
Division  (direct),   of    germ-cells,  11, 

12. 
Dorsal  aorta,  origin  of,  155. 
Driesch,  sea-urchin  egg,  126,  127. 

action  of  nucleus  on  cell,  135. 

theory  of  embryo,  136. 
Dumas,  account  of  cleavage,  48. 

Ear,  162,  163. 

Echinodermata,    isolation    of    blasto- 
mere, 131. 
Echinus,  isolation  of  one-fourth  and 

one-eighth  blastomere,  133. 
Ectoderm,  71,  72. 

ciliation  of,  165. 

organs  from,  159. 
Efferent  branchial  vessels,  153-155. 
Egg,  separation  from  ovary,  15. 

orientation  of,  81. 

rotation  of,  83-85. 

rotation  of  contents,  91,  92. 

in  centrifugal  machine,  92-94. 

of  Rana  fusca,  size  of,  95. 

fragments  of  sea-urchin,  130,  131. 

ctenophor,  fragments  of,  131. 
Egg-axis,  32. 

relation  to  cleavage-plane,  82, 85-87. 
Egg-laying,  Introduction. 
Egg-membranes,  17,  19,  20. 


Egg-membranes,    absorption    of    heat 

by,  20. 
protection  by,  19,  20. 
Egg-nucleus,  migration  of,  13. 
Elastic  plates,  63,  64. 
Embryo,  on  compressed  egg,  98-101. 
Embryonic  ring,  64-66. 
Embryos,  double,  117-119. 
Endoderm,  70,  73. 
Endodermal  cells  of  heart,  151. 
Endothelium  of  heart,  151. 
Endres  and  Walter,  half-embryos,  115, 

116. 
Epigenesis,  125,  126. 
Evolution,  125,  126. 
Eye,  60,  64,  161. 

Facial  nerve,  164. 
Fiedler,  126,  127. 
Field,  H.  H.,  origin  of  pronephros,  155- 

158. 
Fin,  62. 
Flemming,   spermatogenesis  of    SalS/- 

mandra,  5,  6. 
Fore-brain,  62,  100. 
Fiirbringer,  pronephros,  158. 

Ganglia,  spinal,  62. 

Germinal  localization,  125,  126. 

Germ-plasm,  124. 

Gill- arches,  62. 

Gill-plate,  58-60. 

Gill-slits,  62. 

formation  of,  141,  144. 

relation  of  nerves  to,  164. 
Gills,  ciliation  of,  167. 
Glomus,  156,  157. 
Glosso-pharyngeal  nerve,  164. 
Goette,  origin  of  pronephros,  158. 
Gryllotalpa,  2. 

Half-blastula  of  Echinus,  127. 
Half-embryos  of  Echinus,  133. 
Half-larva,  ctenophor,  130. 
Head-kidneys,  156-158. 

veins  from,  153. 
Head-somites,  148. 
Heart,  150-155. 

bending  of  tube,  151. 
Hemiembryo,  anterior,  109. 

lateralis,  107,  108. 

posterior,  109. 
Hemiooholoplasten,  121. 


IXDEX 


189 


Hertwig,  O.,  second  maturation-divi- 
sion, 8,  9. 

rotation  of  egg,  20. 

cross-fertilization  of  sear-urchin,  28. 

polyspermy,  30. 

origin  of  mesoderm,  70. 

formation  of  spina  bifida,  77. 

effect  of  gravity,  90. 

compression  of  egg,  95,  99,  100. 

egg  in  tube,  101. 

law  of  cleavage,  102,  103. 

injury  to  blastomere,  112-115. 

methods  of  injury,  112. 

hemiembryo  lateralis,  113. 

asymmetry  of  embryo,  113,  114. 

criticism  of  Roux,  114,  115, 

revivification  of  injured  blastomere, 
114,  115. 

criticism  of,  121. 

quantitative  division,  127. 

interaction  of  blastomeres,  134. 

effect  of  temperature,  168,  169. 
Heterotypic  division,  5,  6. 
Hind-brain,  160. 
His,  elastic  plates,  63,  64. 

germinal  localization,  125,  126. 
Hoffmann,  pronephros,  158. 
Homoeotypic  division,  6,  7. 
Hydromedusae,  isolation  of  blastomere, 

131. 
Hyla,  spermatozoon  of,  11. 
Hyoid  arch,  145. 

vessel,  153. 
Hyomandibular-cleft,  145. 

Idioplasm,  128. 
Infundibulum,  160,  161. 
Interaction  of  parts,  124,  125. 
Invagination  of  archenteron,  70. 

Isotropy,  87. 

Jordan,  E.  O.,  entrance  of  spermato- 
zoon, 35,  36. 
fertilization,  24. 
median  plane  of  embryo,  42. 
overgrowth  of  blastopore,  68. 

Kolliker,  account  of  cleavage,  49. 
Kupffer,  fertilization,  22. 

Lataste,  cross-fertilization,  26. 
Lateral  line,  164. 
Lens,  162. 


Liver,  origin  of,  141. 

relation  of,  to  heart,  151. 
Lungs,  145. 

Mandibular  vessels,  153,  154. 
Marshall,  origin  of  gill-slits,  144,  145. 

account  of  lungs  and  thyroid  body, 
145. 

origin  of  optic  nerve,  161. 

cranial  nerves,  164. 
Marshall  and  Bles,  pronephros,  158. 
Maturation-divisions,   of  Gryllotalpa, 
2-4. 

of  Salamandra,  7,  9. 

of  frog,  10. 
Maxillary  process,  inferior,  61. 

superior,  61. 
Medulla  oblongata,  160. 
Medullary  folds,  inner,  57-59. 

outer,  57-59. 
Medullary  plate,  73,  158. 

formation  of,  80. 

half,  108. 

length  of,  80. 
Mesenchyme,  148,  149. 
Mesoderm,  69,  71-74. 

early  condition  of,  146. 

extension  of,  ventrally,  146. 

around  blastopore,  147. 

in  region  of  pharynx,  147. 
Mesodermic  somites,  148. 
Methods  of  preservation.  Appendix. 
Meves,  spermatogenesis  of  Salaman- 
dra, 5,  9. 

direct  division  of  germ-cells,  12. 
Mid-brain,  160. 
Mole-cricket,  2. 

Morgan,   T.    H.,   formation  of  spina 
bifida,  77. 

injury  to  blastomere,  120,  121. 

isolation  of  blastomeres  by,  133. 
Morula,  107. 
Mouth,  60,  61. 

Miiller,  origin  of  pronephros,  158. 
Muscle  fibres,  origin  of,  149. 

Nasal  pits,  62. 

Nephrostomes  of  pronephros,  156-158. 

Nerves,  163,  164. 

dorsal  roots,  163. 

ventral  roots,  163. 
Nerve-tube,  bending  of,  160. 

closure  of,  160. 


190 


DEVELOPMENT  OF   THE   FROG'S  EGG 


Nervous  system,  central,  159-161. 
Neural  crest,  159,  160. 
Neural  ridge  or  crest,  163,  164. 
Neurenteric  canal,  138-140. 
Newport,  absorption  of  water  by  egg- 
membranes,  19. 

entrance  of  egg  into  oviduct,  16. 

median  plane  of  embryo,  42. 
Newt,  fertilization  of,  24. 
Notochord,  70,  73,  74. 

of  spina  bifida,  76-78. 

half,  108. 

origin  of,  146. 
Nuclei,  distribution  in  compressed  egg, 

104,  105. 
Nucleus,  control  of  cell  by,  128. 

qualitative  division  of,  129. 
Nussbaum,  entrance  of  egg  into  ovi- 
duct, 16. 

Oil-drops,  43-47. 
Oogenesis,  12. 

and   spermatogenesis,   comparison 
of,  13,  14. 
Optic  lobes,  160. 
Optic  stalk,  162. 
Optic  vesicles,  160,  161. 
Orientation  of  egg,  81. 

Pelobates,  cross-fertilization,  26. 

Pericardium,  151. 

Pfliiger,  cross-fertilization,  26-28. 

median  plane  of  embryo,  42. 

blastopore,  51-53,  56. 

account  of  experiments,  81-89. 

methods,  82. 

conclusions  from  experiments,  87- 
89. 

compression  of  egg,  95. 

conclusions  from  compressed  egg, 
101,  102. 

cleavage-plane    and    embryo-axis, 
108. 
Pharynx,  62,  145. 
Pigment,  distribution  of,  15. 

rotation  of,  83. 
Pineal  body,  160,  161. 
Pituitary  body,  161. 
Plane  of  embryo,  median,  42. 
Pneumogastric  nerves,  164. 
Polar  body,  first,  extrusion  of,  16-18. 

second,  21. 

in  inverted  egg,  91. 


Polarization  of  egg,  88. 
Poles  of  egg,  81. 
Polyspermy,  30. 
Post-anal-gut,  141. 

Postgeneration,    110,    111,    116,    128, 
129. 

of  archenteron.  111. 

of  medullary  folds,  111. 

of  mesoderm.  111. 

of  ectoderm,  111. 

of  whole  embryo,  122. 
Provost,  account  of  cleavage,  48. 
Primitive  groove,  57,  72. 
Primitive  segments,  origin  of,  147. 
Proctodseum,  141,  158. 
Pronephric  capsule,  156. 
Pronephros,  155-158. 
Pro-nucleus,  union,  23. 

apposition  of,  35. 

Hana    arvalis,    cross-fertilization,   27, 

28. 
Rana  esculenta,  spermatozoon  of,  11. 

cross-fertilization,  26-28. 

effect  of  light,  170. 
Rana  fusca,  extrusion  of  polar  body, 
21. 

cross-fertilization,  26-28. 

inversion  after  first  cleavage,  116- 
118. 

effect  of  temperature,  168,  169. 
Rana  temporaria,  egg-laying,  17. 

time  of  egg-laying,  168. 

effect  of  light,  169,  170. 
vom  Rath,  spermatogenesis  of  Gryllo- 
talpa,  2-4. 

spermatogenesis  of  Salamandra,  5, 
7. 

tetrad  formation  in  Salamandra,  8. 

spermatogenesis  of  frog,  10. 

direct  division  of  germ-cells,  12. 
Rauber,  interchange  of  nuclei,  30. 

segmentation,  39. 

effect  of  gravity,  90. 
Reichert,  account  of  cleavage,  49. 
Remak,  segmentation,  38. 

account  of  cleavage,  49. 
Reorganization,  109,  110. 
Retma,  161. 
Robinson,  formation  of  archenteron, 

70. 
Rotation  of  egg,  83-85. 
Roux,  artificial  fertilization,  32. 


INDEX 


191 


Roux  (continued). 

median  plane  of  embryo,  42. 

experiments  with  oil-drops,  43-47. 

spina  bifida,  75. 

centrifugal  machine,  92-94. 

egg  in  tube,  100,  101. 

methods,  106,  107. 

mjur>^  to  blastomere,  106-111. 

cleavage-plane    and    embryo-axis, 
108. 

mosaic  theory,  109,  123,  126. 

whole  embryos,  121. 

subsidiary  hypothesis,  127-129. 

anachronism  in  cleavage,  129. 

part  of  egg  removed,  130. 

qualitative    division    of     nucleus, 
134. 
Rusconi,  cross-fertilization,  24. 

account  of  cleavage,  48. 

Sachs,  law  of  cleavage,  102. 
Salamandra,  isolation  of  blastomere, 

131. 
Salt-solution,  effect  of,  77. 
Schleiden,  49. 

Schnetzler,  effect  of  light,  169. 
Schultze,  M. ,  segmentation,  39. 

account  of  cleavage,  49. 
Schultze,  0.,  formation  of  egg,  12. 

rotation  of  egg,  20. 

polar  bodies,  21. 

origin  of  mesoderm,  71. 

experiments  of,  116-118. 

effect  of  temperature,  169. 
Schwann,  49. 
Sea-urchin,  cross-fertilization,  30. 

isolation  of  blastomeres,  126. 

half  development,  127. 

fragments  of  egg,  130,  131. 
Segmentation,  variations  of,  41. 
Segmentation-cavity,  40,  41,  67,  71. 
Self-differentiation,  123,  124,  126. 
Semiblastula  verticalis,  107,  108. 
Semigastrula  verticalis,  107,  108. 
Semimonila  verticalis,  107,  108. 
Sense-plate,  57-60. 
Sex-cells,  development  of,  1. 
Sinus  venosus,  151. 
Size  of  egg,  95. 
Somatic  layer,  of  mesoderm,  147. 

of  heart,  151. 
Somites,  mesodermic,  148. 

of  head,  148. 


Spallanzani,  egg-laying,  17. 

account  of  cleavage,  47. 
Spermatid,  1. 
Spermatocyte,  1. 
Spermatogenesis,  1,  10. 

salamander  4,  5. 
Spermatogenesis  and  oogenesis,  com- 
parison of,  13,  14. 
Spermatogonia,  1. 
Spermatozoon,  of  frog,  4,  5,  11. 

inheritance  through,  134. 
Spina  bifida,  75-80. 
Splanchnic  layer  of  mesoderm,  147. 

of  heart,  151. 
Star-fish,  cross-fertilization,  30. 
Stomodaeum,  60,  159. 
Strasburger,  action  of  nucleus  on  cell, 

135. 
Suckers,  60,  62,  166. 
Swammerdam,   passage   of  egg   from 
ovary  to  oviduct,  17. 

account  of  cleavage,  47. 
Sylvian  aqueduct,  100. 

TaU,  62. 

Teleostei,  isolation  of  blastomere,  131. 

Temperature,  effect  of,  167-170. 

Tetrad,  3,  4,  8. 

Thyroid  body,  145. 

Toad,     European,    spermatozoon    of, 

11. 
Totipotence,  132,  133. 
Trigeminal  nerve,  164. 
Triton  alpestris,  cross-fertilization,  26, 
28. 
taeniatus,  cross-fertilization,  26,  28. 
Truncus  arteriosus,  153. 

Urodela,  anus  of,  139,  140. 

closure  of  blastopore,  139,  140. 

Vagus,  near  first  somite,  148. 

V.  la  Valette  St.  George,  terminol- 
ogy, 1. 

Ventricle  of  heart,  153. 

Visceral-arches,  145. 

Visceral-slits,  145. 

Vitelline  veins,  151. 

Vitreous  body,  162. 

Vogt,  segmentation,  38. 

de  Vries,  action  of  nucleus  on  cell, 
135. 


192 


DEVELOPMENT  OF  THE   FROG'S   EGG 
Wrinkles  of  egg,  33. 


Weismann,  theory  of  heredity,  14. 

qualitative  nuclear  division,  129. 

qualitative    division    of    nucleus, 
134. 
Wetzel,  double  embryos,  118,  119. 
Whitman,  theory  of  embryo,  136. 
Wichmann,  pronephros,  158. 
Wilson,  E.  B.,  amphioxus,  127. 


Yolk-granules,  absorption  of,  141. 
Yolk-plug,  withdrawal  of,  140. 
Yung,  effect  of  light,  169,  170. 

Ziegler,  embryos,  61. 

Zoja,  isolation  of  blastomeres  by,  132. 


COLUMBIA  UNIVERSITY  BIOLOGICAL  SERIES. 

Designed  for  Independent  Reading  and  as  Text-Boolcs  for  Lecture  and  Laboratory 
Courses  of  Instruction. 

Edited  by  HENRY  FAIRFIELD  OSBORN, 

Da  Costa  Professor  of  Zoology  in  Columbia   University. 


VOL.  I.  FROM  THE  GREEKS  TO  DARWIN.  The  Development  of  the  Evolution 
Idea.  By  HENRY  Fairfield  Osborn,  ScD.  8vo.  Cloth.  Price  $2.00,  net. 
"  This  is  an  attempt  to  determine  the  history  of  Evolution,  its  development  and  that  of  its  ele- 
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Queen's  Quarterly.  — "This  edition  is  really  his  large  five-volume  book  revised  and 
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The  condensation  has  been  achieved  by  cutting  out  all  the  parts  on  histology. 
Dr.  Foster  has  also  unified  the  work  as  a  whole  by  omitting  all  those  theoretical  dis- 
cussions in  the  larger  book  which  made  it  read  like  a  succession  of  articles  from  the 
Journal  of  Physiology.  Not  that  he  has  excised  all  theory:  there  is  still  plenty  of  it : 
but  it  is  here  restrained  and  kept  in  its  proper  perspective  in  relation  to  the  rest  of 
the  book." 

Physician  and  Surgeon.  — "Nothing  can  be  said  in  praise  of  this  great  work  which 
is  undeserved.  Every  page  bears  witness  to  the  zeal  and  devotion  to  scientific  truth 
which  have  made  it  a  standard  authority  upon  the  subject  of  which  it  treats." 

Professor  S.  W.  Williston,  University  of  Kansas^  Lawrence^  Kansas.  —  "I  .shall 
be  very  glad  to  recommend  this  edition  to  my  students.  I  have  regretted  much  that 
the  only  full  and  reliable  edition  in  the  late  past  has  been  the  original  English,  rather 
too  full  in  some  parts  and  too  expensive  for  the  ordinary  medical  student.  I  regard 
Foster  as  the  only  physiology  for  the  medical  student." 


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A  TEXT- BOOK  OF 

SPECIAL  PATHOLOGICAL  ANATOMY, 


ERNST   ZIEGLER, 

Pi'ofessor  of  Pathology  in  the  University  of  Freiburg, 

TRANSLATED  AND   EDITED   FROM  THE   EIGHTH   GERMAN   EDITION   BY 

DONALD  MacALISTER,   M.A.,  M.D., 

Linacre  Lecturer  of  Physic,  and  Tutor  of  St.  John^s  College,  Cambridge, 

AND 

HENRY  W.   CATTELL,   M.A.,  M.D., 

Demonstrator  of  Morbid  Anatomy  in  the  University  of  Pennsylvania. 

VOL.  I.     SECTIONS    I.-Vin. 
8vo.      Cloth.      Price  $4.00,  net. 


FROM  THE  TRANSLATORS'   PREFACE. 

Since  the  publication,  in  1884,  of  the  first  English  edition  of  Ziegler's  Special 
Pathological  Anatomy,  great  advances  have  been  made  in  our  knowledge  of  its 
subject-matter.  These  have  been  duly  embodied  in  the  five  successive  German 
editions  that  have  appeared  in  the  meanwhile.  The  work  has  accordingly  been 
so  altered  and  enlarged  that  in  preparing  a  third  English  edition  we  have  had 
entirely  to  rewrite  the  text,  and  to  recast  the  bibliographical  and  other  supple- 
mentary portions.  The  number  of  pathological  papers  and  monographs,  to 
which  reference  might  fitly  be  made,  is  now  so  great  that  only  the  more  recent 
and  important  can  be  dealt  with.  But  the  student  of  historical  tastes  will  find 
ample  references  to  the  earlier  Hterature  in  the  previous  English  editions  ;  and, 
by  omitting  them  in  this,  much  valuable  space  has  been  gained. 

The  second  volume,  containing  the  sections  on  the  alimentary  tract  with  the 
liver  and  pancreas,  the  respiratory  and  genito-urinary  systems,  the  eye,  and  the 
ear,  is  already  in  the  press  and  will  shortly  be  published. 


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IN    PREPARATION. 

A  SYSTEM  OF  GYNAECOLOGY. 

BY  MANY    WRITERS. 

EDITED  BY 

THOMAS   CLIFFORD   ALLBUTT, 

MJ\.,  M.D..  LL.D.,  F.R.C.P..  F.R.S..  F.L.S.,  F.S.A., 

Regius  Professor  of  Physic  in  the  University  of  Cambridge,  Fellow  of  Gonville 
and  Caius  College, 

AND 

WILLIAM   SMOULT    PLAYFAIR, 

M.D..  LL.D.,  F.R.C.P., 

Professor  of  Obstetric  Medicine  at  King's  College,  and  Physician  to  Women  and  Children 
at  King's  College  Hospital. 


MEDIUM  OCTAVO,  ABOUT  looo  PAGES,  WITH  MANY  ILLUSTRATIONS. 

Cloth,  $6.00,  net ;    Half  Leather,  $7.00,  net. 

OR   TO    SUBSCRIBERS    TO    "A    SYSTEM    OF    MEDICINE,"    EDITED    BY    THOMAS 

CLIFFORD  ALLBUTT, 

Cloth,  $5.00,  net;    Half  Leather,  $6.00,  net. 


CONTENTS. 


Development  of  Modern  Gynaecology,  J/.  Haiidfield  Jones. — Anatomy  of  the  Female 
Genital  Organs,  D.  Berry  Hart.  —  Malformations  of  the  Genital  Organs  in  Women, 
/.  William  Ballantyne.  —  Etiology  of  Diseases  of  the  Genital  Organs  in  Women,  //'. 
Balls-Headley.  —  Diagnosis  in  Gynaecology,  Robert  Boxall.  —  Inflammatory  Diseases 
of  the  Uterus,  A.  H.  Freeland  Barbour.  —  The  Nervous  System  in  Relation  to  Gynae- 
cology, W.  S.  Play  fair.  —  Sterility,  Henry  Gervis.  —  Gynaecological  Therapeutics, 
Amand  Routh.  —  Electricity  in  Gynaecology,  Robert  Milne  Murray.  —  Diseases  of 
Menstruation, /<!7//;/  Halliday  Groom.  —  Diseases  of  the  Vulva  and  Vagina,  William  J. 
Smyly.  —  Displacements  of  the  Uterus,  Alexander  Russell  Simpson.  —  Morbid  Condi- 
tions of  Female  Genitals,  George  Ernest  Herinan.  —  Extra-Uterine  Gestation, /<?/;« 
Bland  Sutton.  —  Pelvic  Inflammations,  Charles  James  Cullingworth.  —  Pelvic  Haema- 
tocele,  William  Over  end  Priestley.  —  Tumours  of  the  Uterus,/^.  W.  N.  Haultaii .  - 
Malignant  Diseases  of  the  Genital  Organs  in  Women,  W.J.  Sinclair.  —  Hysterectomy 
and  Allied  Operations,/.  Knowsley  Thornton.  —  Plastic  Gynaecological  Operations, 
John  Phillips.  —  Diseases  of  the  Fallopian  Tubes,  Alban  Doran.  —  Diseases  of  the 
Ovary,  W^.  S.  A.  Griffith.  —  Ovariotomy  and  Allied  Operations,  /.  Greig  Smith.  — 
Chronic  Inversion  of  the  Uterus,  Edward  Malins.  —  Diseases  of  the  Female  Bladder 
and  Urethra,  Henry  Morris. 


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3 


MANUAL  OF  MIDWIFERY. 

BY 

W.    E.    FOTHERGILL, 

M.A.,  B.Sc,  M.B.,  CM.,  Etc.,  Etc. 

WITH  DOUBLE  COLOURED  PLATE  AND   SIXTY-NINE  ILLUSTRATIONS 

IN   THE   TEXT. 

i2nio.     Cloth.     Price  $2.25,  net. 


CONTENTS. 


Chapters    i  and   2.  STRUCTURE    AND     FUNCTIONS     OF    THE    FEMALE 

REPRODUCTIVE  ORGANS. 

Chapters    3   to    6.  PREGNANCY,  DIAGNOSIS  AND  DEVELOPMENT. 

Chapters    7   to    9.  PATHOLOGY  OF  PREGNANCY. 

Chapters  10   to  14.  LABOUR. 

Chapter   15.  MORBID  LABOUR. 

Chapter   16.  PRiETERNATURAL  LABOUR. 

Chapters  17   to  19.  COMPLEX  LABOUR. 

Chapters  20  and  21.  OBSTETRIC  OPERATIONS. 

Chapter   22.  THE  PUERPERIUM. 

Chapter   23.  HYGIENE  OF  INFANCY. 


COMMENTS. 


"  An  exceedingly  interesting  and  instructive  volume  by  one  of  the  most 
advanced  writers  in  the  specialty  treated."  —  The  Medical  Examiner. 

"  A  well  prepared  and  very  interesting  work.  Will  prove  invaluable  to  any 
one  interested  in  this  subject,  and  is  well  calculated  to  answer  the  purpose  of  the 
general  practitioner  who  desires  to  brighten  up  on  the  important  features.  We 
commend  it  to  the  examination  of  all  interested  in  the  subject." — Charlotte 
Medical  Journal. 

"  Few  books  are  so  well  worth  the  money  as  is  this  terse  and  well-written 
book.  It  is  one  of  the  few  modern  books  that  bear  the  stamp  of  individuality 
and  originality.  The  entire  field  is  covered.  A  production  that  will  make  every 
practitioner  think  who  reads  it,  and  will  make  him  a  better  obstetrician."  —  The 
Medical  Council. 


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4 


ATLAS 

OF  THE 

FERTILIZATION  and  KARYOKINESIS 

OF  THE   OVUM 

By  EDMUND   B.  WILSON,  Ph.D., 

Professor  of  Invertebrate  Zoology  in  Columbia  College 


WITH  THE  CO-OPERATION  OF 


EDWARD  LEAMING,  M.D.,  F.R.P.S., 

Instructor  in  Photography  at  the  College  of  Physicians  and  Surgeons,  Columbia  College 

WITH  TEN   PHOTOGRAPHIC  PLATES  AND   NUMEROUS   DIAGRAMS 

Extra  8vo.    Cloth,    pp.  vH  +  32.    $4.00,  net. 


This  work  comprises  forty  figures,  photographed  from  nature  by  Dr.  Learning  from  the 
preparations  of  Professor  Wilson  at  an  enlargement  of  one  thousand  diameters,  and 
mechanically  reproduced  by  the  gelatine  process,  without  retouching,  by  Edward  Bierstadt 
of  New  York.  The  plates  are  accompanied  by  an  explanatory  text,  giving  a  general  intro- 
duction to  the  subject  for  the  use  of  students  and  general  readers,  a  detailed  description 
of  the  photographs,  and  over  sixty  text-figures  from  camera-drawings. 

It  is  the  object  of  this  atlas  to  place  before  students  and  teachers  of  biology  a  practically 
continuous  series  of  figures  photographed  directly  fi-om  nature,  to  illustrate  the  principal 
phenomena  in  the  fertilization  and  early  development  of  the  animal  egg.  The  new  science 
of  cytology  has  in  the  course  of  the  past  two  decades  brought  forsvard  discoveries  relating 
to  the  fertilization  of  the  egg  and  the  closely  related  subject  of  cell-division  (karyokinesis) 
that  have  called  forth  on  the  part  of  Weismann  and  others  some  of  the  most  important 
and  suggestive  discussions  of  the  post-Darwinian  biology.  These  discoveries  must  in 
some  measure  be  dealt  with  by  every  modern  text-book  of  morphology  or  physiology,  yet 
they  belong  to  a  region  of  observation  inaccessible  to  the  general  reader  or  student,  since 
it  can  only  be  approached  by  means  of  a  refined  histological  technique  applied  to  special 
objects  not  ordinarily  available  for  practical  study  or  demonstration.  A  knowledge  of  the 
subject  must  therefore,  in  most  cases,  be  acquired  from  text-books  in  which  drawings  are 
made  to  take  the  place  of  the  real  object.  But  no  drawing,  however  excellent,  can  convey 
an  accurate  mental  picture  of  the  real  object.  It  is  extremely  difficult  for  even  the  most 
skilful  draughtsman  to  represent  in  a  drawing  the  exact  appearance  of  protoplasm  and  the 
delicate  and  complicated  apparatus  of  the  cell.  It  is  impossible  adequately  to  reproduce 
the  drawing  in  a  black-and-white  text-book  figure.  Every  such  figure  must  necessarily  be  in 
some  measure  schematic  and  embodies  a  considerable  subjective  element  of  interpretation. 

The  photograph,  whatever  be  its  shortcomings  (and  no  photograph  can  do  full  justice 
to  nature),  at  least  gives  an  absolutely  faithful  representation  of  what  appears  under  the 
microscope ;  it  contains  no  subjective  element  save  that  involved  in  the  focussing  of  the 
instrument,  and  hence  conveys  a  true  mental  picture.  It  is  hoped,  therefore,  that  the  pres- 
ent work  may  serve  a  useful  purpose,  especially  by  enabling  teachers  of  biology  to  place 

7 


before  their  students  a  series  of  illustrations  whose  fidelity  is  beyond  question,  and  which 
may  serve  as  a  basis  for  either  elementary  or  advanced  work  in  this  direction. 

Following  is  a  partial  list  of  the  points  clearly  shown  in  the  present  series:  The 
ovarian  egg,  with  germinal  vesicle,  germinal  spot  and  chromatin-network ;  the  polar 
amphiaster  with  the  "  Vierergruppen  "  or  quadruple  chromosome-groups ;  the  unfertilized 
egg,  after  extrusion  of  the  polar  bodies;  entrance  of  the  spermatozoon,  the  entrance-cone; 
rotation  of  the  sperm-head,  origin  of  the  sperm-aster  from  the  middle-piece,  growth  of  the 
astral  rays;  conjugation  of  the  germ-nuclei,  extension  and  division  of  the  sperm-aster; 
formation  of  the  cleavage-nucleus ;  the  attraction-spheres  in  the  resting-cell ;  formation  of 
the  cleavage-amphiaster,  origin  of  the  spindle-fibres  and  chromosomes ;  division  of  the 
chromosomes,  separation  of  the  daughter-chromosomes;  structure  and  growth  of  the 
astrosphere ;  degeneration  of  the  spindle ;  formation  of  the  "  Zwischenkorper  "  ;  origin  of 
the  chromatic  vesicles  from  the  chromosomes;  reconstruction  of  the  daughter-nuclei; 
cleavage  of  the  ovum ;  the  two-celled  stage  at  several  periods  showing  division  of  the 
archoplasm-mass, "  attraction-spheres  "  in  the  resting-cell,  formation  of  the  second  cleavage- 
amphiasters. 

FROM  THE  PRESS 

"A  work  which  is  an  honor  to  American  scholarship."  —  Philadelphia  Evening  Tele- 
graph. 

"  Professor  Wilson  has  rendered  a  great  service  to  teachers  and  students  in  the  publica- 
tion of  the  splendid  series  of  micro-photographs  of  these  different  processes.  These  are 
accompanied  by  an  admirably  lucid  text."  —  The  Dial. 

"  It  is  not  often  that  one  is  permitted  to  examine  a  piece  of  work  which  is  done,  in  all 
respects,  on  an  ideal  standard,  as  this  is.  .  .  .  It  is  safe  to  say  that  the  whole  area  engaged 
in  the  fertilization  and  division  of  the  ovum  has  never  been  shown  or  the  forces  traced 
with  such  precision  before." —  The  Independent. 

"  Every  biologist  owes  the  greatest  gratitude  to  the  authors  and  publishers  of  this 
beautiful  volume;  and  only  those  who  have  labored  themselves  to  make  good  photo- 
graphic plates  from  specimens  exhibiting  karyokinesis,  can  appreciate  the  wonderful 
delicacy  of  the  results."  —  Natural  Science. 

"  This  work  is  of  a  very  high  order,  and  both  by  its  merit  and  its  opportuneness  is  a 
noteworthy  contribution  to  science.  .  .  .  The  pictures  obtained  represent  the  highest 
perfection  of  micro-photography  yet  reached,  especially  as  applied  to  protoplasmic  struct- 
ures. ...  To  the  whole  is  prefixed  an  abundantly  illustrated  "General  Introduction" 
in  which  Professor  Wilson  gives  a  summary  of  our  present  knowledge  of  our  history  of 
the  ovum,  so  far  as  it  has  any  bearing  on  the  problems  of  fertilization.  It  would  be  very 
difficult  to  surpass  this  introduction,  owing  to  its  felicitous  combination  of  terseness, 
clearness,  and  completeness.  The  work  takes  its  place  at  once  as  a  classic,  and  is  certainly 
one  of  the  most  notable  productions  of  pure  science  which  have  appeared  in  America. 
It  will  be  valuable  to  every  biologist,  be  he  botanist  or  zoologist,  be  he  investigator  or 
teacher."  —  Science. 


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JUST  READY 

An  Atlas  of  Nerve  Cells 

BY 

M.  ALLEN  STARR,  M.D.,  Ph.D., 

professor  of  Diseases  of  the  Mind  and  Nervous  System,  College  of  Physicians  and  Surgeons 

Medical  Department  of  Columbia  College  ;    Consulting  Neurologist  to  the  Presbyterian 

and  Orthopadic  Hospitals,  and  to  the  New  York  Eye  and  Ear  Infirmary 

WITH  THE   CO-OPERATION  OF 

0.  S.  STRONG,  Ph.D.,  and  EDWARD  LEAMTNG,  M.D. 
Extra  4to.    Cloth.    $10.00,  net. 

UNIFORM  WITH  DR.  WILSON'S  "ATLAS  OF  THE  FERTILIZATION  OF  THE  OVUM  " 

52  PLATES.      8   FIGURES. 

It  is  the  object  of  this  atlas  to  present  to  students  and  teachers  of  histology  a  series  of 
photographs  showing  the  appearance  of  the  cells  which  form  the  central  nervous  system, 
as  seen  under  the  microscope.  These  photographs  have  been  made  possible  by  the  use 
of  the  method  of  staining  invented  by  Professor  Camillo  Golgi  of  Turin.  This  method 
has  revealed  many  facts  hitherto  unknown,  and  has  given  a  conception  of  the  structure  and 
connections  of  the  nerve  cells  both  novel  and  important. 

In  the  most  recent  text-books  of  neurology  and  in  the  atlas  of  Golgi  these  facts  have 
been  shown  by  drawings  and  diagrams.  But  all  such  drawings  are  necessarily  imperfect 
and  involve  a  personal  element  of  interpretation.  It  has  seemed  to  me,  therefore,  that  a 
series  of  photographs  presenting  the  actual  appearance  of  neurons  under  the  microscope 
would  be  not  only  of  interest  but  also  of  service  to  students.  The  Golgi  method  lends 
itself  very  readily  to  the  photographic  process,  for  the  cell,  with  its  dendrites  and  neuraxon, 
is  stained  black  upon  a  light  yellowish  ground,  and  thus  is  capable  of  giving  a  sharp  pict- 
ure. In  the  preparation  of  this  Atlas  I  have  had  the  co-operation  of  Dr.  O.  S.  Strong,  who 
has  cut  and  stained  the  specimens,  and  of  Dr.  Edward  Leaming,  whose  skill  in  photogra- 
phy has  made  this  work  possible.  Dr.  Strong  has  been  able  to  produce  remarkably  suc- 
cessful sections  of  the  various  parts  of  the  nervous  system,  both  brain  and  spinal  cord,  and 
has  made  some  valuable  modifications  of  Golgi's  methods.  He  has  contributed  a  section 
upon  the  technique,  containing  many  original  and  important  suggestions.  In  the  art  of 
photographing  microscopic  specimens  Dr.  Leaming  has  been  particularly  successful.  It 
can  be  readily  imagined  that  the  difficulties  of  obtaining  a  clear  picture  focussed  in  one 
plane  upon  the  photographic  plate  are  at  times  almost  insuperable,  the  microscopist 
ordinarily  bringing  various  planes  into  his  vision  by  the  aid  of  the  fine  adjusting  screw  of 
the  instrument.  By  care  in  the  selection  of  specimens,  by  ingenious  contrivances  to 
ensure  a  perfect  focussing,  and  by  the  use  of  various  methods' adapted  to  each  emergency, 
Dr.  Leaming  has  succeeded  where  others  have  failed.  He  has  contributed  a  section  of 
much  value  upon  the  photographic  technique.  The  photographs  have  been  reproduced 
in  a  painstaking  manner  by  Mr.  Edward  Bierstadt,  whose  process  of  autotyping  has  been 
selected  after  a  careful  comparison  with  other  methods  of  reproduction ;'  and  it  can  be 
justly  said  that  they  show  every  detail  of  the  original  photographs. 

In  presenting  this  Atlas  I  have  not  attempted  to  write  an  exhaustive  account  of  nervous 
histology,  but  rather  to  present  a  brief  review  of  the  essential  facts  so  far  as  they  can  be 
seen  by  the  aid  of  the  Golgi  stain,  and  to  show  how  these  facts  aid  in  the  knowledge  of 
nervous  action.  I  may  be  permitted,  however,  to  point  out  that  this  atlas  is  based  mainly 
upon  preparations  from  the  human  nervous  system ;  that  it  not  only  includes  the  spinal 
cord,  cerebellum,  and  brain  cortex,  which  have'been  studied  by  Golgi,  Cajal,  Van  Gehuch- 
ten,  Retzius,  and  Lenhossek,  but  also  presents  original  studies  of  the  corpora  quadrigemina, 
optic  thalamus,  and  lenticular  and  caudate  nuclei,  and  is  thus  quite  complete  in  its  scope. 
It  is  my  intention  at  some  future  time  to  issue  another  volume  which  will  include  the 
peripheral  nerves  and  their  terminations  and  the  organs  of  sense. 


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